Return empty list of measurable outputs from UnmeasurableVariables instead of None

Author: ricardoV94

pymc/logprob/abstract.py

@@ -174,7 +174,7 @@ def _get_measurable_outputs_RandomVariable(op, node):
 
 
 def noop_measurable_outputs_fn(*args, **kwargs):
-    return None
+    return []
 
 
 def assign_custom_measurable_outputs(

pymc/tests/distributions/test_logprob.py

@@ -325,7 +325,7 @@ def test_ignore_logprob_basic():
     assert isinstance(new_x.owner.op, Normal)
     assert type(new_x.owner.op).__name__ == "UnmeasurableNormalRV"
     # Confirm that it does not have measurable output
-    assert get_measurable_outputs(new_x.owner.op, new_x.owner) is None
+    assert get_measurable_outputs(new_x.owner.op, new_x.owner) == []
 
     # Test that it will not clone a variable that is already unmeasurable
     new_new_x = ignore_logprob(new_x)