Fix residual NotNaN for null partition values

[BharatDeva]

Rationale for this change

ResidualVisitor.visit_not_nan currently treats a partition value of None as AlwaysFalse(). That is inconsistent with the normal evaluator behavior and with Iceberg Java semantics: None is not NaN, so NotNaN(None) should evaluate to true.

This PR addresses only that residual-evaluator edge case from #3498. The stricter metrics-evaluator items discussed in the issue are already covered by other active work.

Are these changes tested?

Yes. I updated the existing residual evaluator expectations for both double and float identity partitions and ran the targeted test file in a WSL-native checkout:

pytest tests/expressions/test_residual_evaluator.py -q

Result:

11 passed in 0.05s

Additional checks:

ruff check pyiceberg/expressions/visitors.py tests/expressions/test_residual_evaluator.py
git diff --check

Both passed.

Are there any user-facing changes?

Yes. Residual evaluation for NotNaN now treats null partition values as not-NaN instead of false. Please add the changelog label if this behavior change should appear in release notes.