Fix duplicate variable in null-space condition of _calc_qnull

[Lidang-Jiang]

Summary

• Fix a typo in _calc_qnull() where if λΣ > 0 or λΣ > 0: should be if λΣ > 0 or λm > 0:.
• This bug caused null-space motion to be skipped entirely when only km (manipulability maximisation) was enabled without kq (joint limit avoidance).
• Added 5 regression tests for _calc_qnull covering all gain combinations.

Fixes #499

Root Cause

In roboticstoolbox/robot/IK.py, the function _calc_qnull has a condition that gates null-space motion calculation:

# Before (buggy):
if λΣ > 0 or λΣ > 0:  # λΣ checked twice, λm never checked

# After (fixed):
if λΣ > 0 or λm > 0:  # correctly checks both gains

When kq=0 (so λΣ=0) and km>0 (so λm>0), the buggy condition evaluates to False, causing the null-space projection to be skipped and returning a zero vector instead of the manipulability gradient projected into the null space.

Before (with bug): qnull is all zeros when only km > 0

============================================================
Issue #499: Null Space Calculation Bug in _calc_qnull
============================================================

--- Test 1: Both gains zero (kq=0, km=0) ---
qnull = [0. 0. 0. 0. 0. 0. 0.]
All zeros: True

--- Test 2: Only km > 0 (kq=0, km=1) [THE BUG CASE] ---
qnull = [0. 0. 0. 0. 0. 0. 0.]
All zeros (BUG if True): True

--- Test 3: Only kq > 0 (kq=1, km=0) ---
qnull = [0. 0. 0. 0. 0. 0. 0.]
All zeros: True

--- Test 4: Both gains positive (kq=1, km=1) ---
qnull = [-0.00356543 -0.00089796 -0.00055962  0.00013665  0.00314415 -0.00055438
 -0.00219734]
All zeros: False

============================================================
Expected behavior:
  Test 2 should return NON-ZERO qnull (manipulability gradient)
  With the bug, Test 2 returns ALL ZEROS
============================================================

After (fixed): qnull is non-zero when km > 0

============================================================
Issue #499: Null Space Calculation Bug in _calc_qnull
============================================================

--- Test 1: Both gains zero (kq=0, km=0) ---
qnull = [0. 0. 0. 0. 0. 0. 0.]
All zeros: True

--- Test 2: Only km > 0 (kq=0, km=1) [THE BUG CASE] ---
qnull = [-0.00356543 -0.00089796 -0.00055962  0.00013665  0.00314415 -0.00055438
 -0.00219734]
All zeros (BUG if True): False

--- Test 3: Only kq > 0 (kq=1, km=0) ---
qnull = [0. 0. 0. 0. 0. 0. 0.]
All zeros: True

--- Test 4: Both gains positive (kq=1, km=1) ---
qnull = [-0.00356543 -0.00089796 -0.00055962  0.00013665  0.00314415 -0.00055438
 -0.00219734]
All zeros: False

============================================================
Expected behavior:
  Test 2 should return NON-ZERO qnull (manipulability gradient)
  With the bug, Test 2 returns ALL ZEROS
============================================================

Unit test results (5 new tests, all passing)

============================= test session starts ==============================
platform linux -- Python 3.11.14, pytest-9.0.2, pluggy-1.6.0 -- /home/devuser/workspace/public-oss/embodied-robotics/myenv-rtb-499/bin/python
cachedir: .pytest_cache
rootdir: /home/devuser/workspace/public-oss/embodied-robotics/robotics-toolbox-python
configfile: pyproject.toml
collecting ... collected 41 items / 36 deselected / 5 selected

tests/test_IK.py::TestCalcQnull::test_qnull_both_gains_positive PASSED   [ 20%]
tests/test_IK.py::TestCalcQnull::test_qnull_both_gains_zero PASSED       [ 40%]
tests/test_IK.py::TestCalcQnull::test_qnull_km_only PASSED               [ 60%]
tests/test_IK.py::TestCalcQnull::test_qnull_km_only_matches_both_when_kq_inactive PASSED [ 80%]
tests/test_IK.py::TestCalcQnull::test_qnull_kq_only PASSED               [100%]

======================= 5 passed, 36 deselected in 1.07s =======================

Full IK test suite (no regressions; 5 QP failures are pre-existing due to missing qpsolvers dependency)

============================= test session starts ==============================
platform linux -- Python 3.11.14, pytest-9.0.2, pluggy-1.6.0 -- /home/devuser/workspace/public-oss/embodied-robotics/myenv-rtb-499/bin/python
cachedir: .pytest_cache
rootdir: /home/devuser/workspace/public-oss/embodied-robotics/robotics-toolbox-python
configfile: pyproject.toml
collecting ... collected 41 items

tests/test_IK.py::TestIK::test_IK_GN1 PASSED                             [  2%]
tests/test_IK.py::TestIK::test_IK_GN2 PASSED                             [  4%]
tests/test_IK.py::TestIK::test_IK_GN3 PASSED                             [  7%]
tests/test_IK.py::TestIK::test_IK_LM1 PASSED                             [  9%]
tests/test_IK.py::TestIK::test_IK_LM2 PASSED                             [ 12%]
tests/test_IK.py::TestIK::test_IK_LM3 PASSED                             [ 14%]
tests/test_IK.py::TestIK::test_IK_NR1 PASSED                             [ 17%]
tests/test_IK.py::TestIK::test_IK_NR2 PASSED                             [ 19%]
tests/test_IK.py::TestIK::test_IK_NR3 PASSED                             [ 21%]
tests/test_IK.py::TestIK::test_IK_NR4 PASSED                             [ 24%]
tests/test_IK.py::TestIK::test_IK_NR5 PASSED                             [ 26%]
tests/test_IK.py::TestIK::test_IK_NR6 PASSED                             [ 29%]
tests/test_IK.py::TestIK::test_IK_NR7 PASSED                             [ 31%]
tests/test_IK.py::TestIK::test_IK_NR8 PASSED                             [ 34%]
tests/test_IK.py::TestIK::test_IK_QP1 FAILED                             [ 36%]
tests/test_IK.py::TestIK::test_IK_QP2 FAILED                             [ 39%]
tests/test_IK.py::TestIK::test_IK_QP3 FAILED                             [ 41%]
tests/test_IK.py::TestIK::test_ets_ikine_GN1 PASSED                      [ 43%]
tests/test_IK.py::TestIK::test_ets_ikine_GN2 PASSED                      [ 46%]
tests/test_IK.py::TestIK::test_ets_ikine_LM1 PASSED                      [ 48%]
tests/test_IK.py::TestIK::test_ets_ikine_LM2 PASSED                      [ 51%]
tests/test_IK.py::TestIK::test_ets_ikine_NR1 PASSED                      [ 53%]
tests/test_IK.py::TestIK::test_ets_ikine_NR2 PASSED                      [ 56%]
tests/test_IK.py::TestIK::test_ets_ikine_QP1 FAILED                      [ 58%]
tests/test_IK.py::TestIK::test_ets_ikine_QP2 FAILED                      [ 60%]
tests/test_IK.py::TestIK::test_ik_gn PASSED                              [ 63%]
tests/test_IK.py::TestIK::test_ik_lm_chan PASSED                         [ 65%]
tests/test_IK.py::TestIK::test_ik_lm_sugihara PASSED                     [ 68%]
tests/test_IK.py::TestIK::test_ik_lm_wampler PASSED                      [ 70%]
tests/test_IK.py::TestIK::test_ik_nr PASSED                              [ 73%]
tests/test_IK.py::TestIK::test_iter_iksol PASSED                         [ 75%]
tests/test_IK.py::TestIK::test_sol_print1 PASSED                         [ 78%]
tests/test_IK.py::TestIK::test_sol_print2 PASSED                         [ 80%]
tests/test_IK.py::TestIK::test_sol_print3 PASSED                         [ 82%]
tests/test_IK.py::TestIK::test_sol_print4 PASSED                         [ 85%]
tests/test_IK.py::TestIK::test_sol_print5 PASSED                         [ 87%]
tests/test_IK.py::TestCalcQnull::test_qnull_both_gains_positive PASSED   [ 90%]
tests/test_IK.py::TestCalcQnull::test_qnull_both_gains_zero PASSED       [ 92%]
tests/test_IK.py::TestCalcQnull::test_qnull_km_only PASSED               [ 95%]
tests/test_IK.py::TestCalcQnull::test_qnull_km_only_matches_both_when_kq_inactive PASSED [ 97%]
tests/test_IK.py::TestCalcQnull::test_qnull_kq_only PASSED               [100%]

=========================== short test summary info ============================
FAILED tests/test_IK.py::TestIK::test_IK_QP1 - ImportError: the package qpsolvers is required
FAILED tests/test_IK.py::TestIK::test_IK_QP2 - ImportError: the package qpsolvers is required
FAILED tests/test_IK.py::TestIK::test_IK_QP3 - ImportError: the package qpsolvers is required
FAILED tests/test_IK.py::TestIK::test_ets_ikine_QP1 - ImportError: the package qpsolvers is required
FAILED tests/test_IK.py::TestIK::test_ets_ikine_QP2 - ImportError: the package qpsolvers is required
========================= 5 failed, 36 passed in 2.70s =========================

Test Plan

• Verify _calc_qnull returns zero vector when both kq=0 and km=0
• Verify _calc_qnull returns non-zero vector when km>0 and kq=0 (the Null Space Calculation Bug in _calc_qnull method #499 regression case)
• Verify _calc_qnull works correctly when kq>0 and km=0
• Verify _calc_qnull works correctly when both kq>0 and km>0
• Verify km-only result matches both-gains result when joints are far from limits
• Full IK test suite passes with no regressions (5 QP failures are pre-existing)