Fix TypeError when dist.metadata returns None on Python 3.15

[Fridayai700]

Summary

Fixes TypeError: argument of type 'NoneType' is not a container or iterable on Python 3.15, where dist.metadata returns None for missing/empty METADATA files instead of an empty PackageMetadata object.

Fixes #530

Changes

• has_valid_metadata(): added dist.metadata is not None guard before the "Name" in dist.metadata check
• Added test_has_valid_metadata_none regression test using a mock dist with metadata=None

Test plan

• New test verifies has_valid_metadata returns False when dist.metadata is None
• Existing test_invalid_metadata continues to pass

[kemzeb]

I currently believe that the changes made to importlib.metadata for 3.15 are a regression, so I rather wait to see what decision the Python maintainers will make here. I'll close this for now, but if they decide to continue on with these changes I'll reopen.