Fix `None` assignments with fine-grained cache (#11574)
Problem: When using the fine-grained cache, we raise an assertion in
some situations when an attribute is defined as `None`.
Solution: Remove truthiness check to ensure that `None` is an allowed
type.
Fixes: https://github.com/python/mypy/issues/8682
Fixes: https://github.com/python/mypy/issues/11456
