PolymorphicQuerySet: Prevent NoneType error in test___lookup

[hll1682003]

Description:

This PR ensures that test___lookup only processes non-None source_expression values to prevent potential NoneType errors. The change adds a simple condition to check if source_expression is None before passing it to test___lookup.

Changes:

Added a None check for source_expression before calling test___lookup.
Why this is needed:

• Prevents potential NoneType errors when evaluating query expressions.
• Ensures test___lookup is only invoked with valid values.
• Our unit tests start failing after migrating from Django 5.0.10 to 5.1.5 and start showing "AttributeError: 'NoneType' object has no attribute 'name'"
• Inspired by this fix, which addressed a similar issue in django-polymorphic.