Keep wildcard keys when a mapped value is null or empty

[Cherith1222]

Proposed changes

Wildcard marshalling treated a null value, or an empty list, as the end of the mapping. Keys that had not been visited yet were dropped. List.format(None) is [], so an empty list compared equal to the old end sentinel. The scan now ends only when no further key remains. A null value is still included unless skip_none is set, and an empty list is kept.

Fixes #409
Fixes #283

Cherith reviewed this diff and confirmed it with the person who helped draft it.
Drafted with Cursor. Model: Grok 4.7.

Types of changes

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

• I have read the guidelines for contributing
• All unit tests pass on my local version with my changes
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation (if appropriate)

Further comments

Windows, Python 3.13. pytest tests --benchmark-skip (the same skip inv test uses): 1233 passed, 6 skipped. tzdata was installed so zoneinfo can resolve CET on Windows. Benchmarks were not run. tox was not run; this machine does not have the other Python versions in the matrix.