Fixed unused_gens for zero polynomial

Author: oscarbenjamin

src/flint/flint_base/flint_base.pyx

@@ -959,7 +959,7 @@ cdef class flint_mpoly(flint_elem):
             ('x0', 'x3')
         """
         names = self.context().names()
-        return tuple(names[i] for i, x in enumerate(self.degrees()) if not x)
+        return tuple(names[i] for i, x in enumerate(self.degrees()) if x <= 0)
 
     def project_to_context(self, other_ctx, mapping: dict[str | int, str | int] = None):
         """

src/flint/test/test_all.py

@@ -3267,6 +3267,8 @@ def quick_poly():
         )
     new_poly = quick_poly().project_to_context(ctx1)
     assert ctx1.drop_gens(new_poly.unused_gens()) == ctx
+    assert quick_poly().unused_gens() == ()
+    assert (0*quick_poly()).unused_gens() == ('x0', 'x1')
     assert new_poly.project_to_context(ctx) == quick_poly()
 
     new_poly = quick_poly().project_to_context(ctx2)