Honor false-valued sort comparators

[harbinresearcher]

Fixes #316.

sort silently ignores a callable comparator whose truth value is false. The same presence checks also bypass the documented comparator/key conflict when either supplied callable is false-valued.

Use explicit None checks for the two guards. Six regression cases cover ascending/reverse behavior with a false-valued comparator, all three false-valued conflict combinations, in-place identity, and a false-valued key supplied alone. Include a changelog entry and the contributor credit required by CONTRIBUTING.

Validation:

• Before the fix, five new cases fail and the independent-key control passes.
• Windows CPython 3.12.14: pristine 2476 passed; patched 2482 passed; both 100% configured coverage.
• Linux CPython 3.10.22, 3.11.17, 3.12.15, 3.13.16 and 3.14.8: pristine 2476 passed and patched 2482 passed on every version, all with 100% configured coverage.
• Official inv ci on Linux 3.12.15 passed: sdist/wheel build, Sphinx with warnings treated as errors, Ruff format/lint, mypy, generated chaining-type consistency, and all tests. Windows mypy and git diff --check passed too.

Disclosure: this contribution was prepared and checked by an AI coding agent. The reproduction comes from local source inspection, not a reported production incident. Automated code review is not human maintainer approval.

[coveralls]

coverage: 100.0%. remained the same — harbinresearcher:codex/honor-falsey-sort-comparators into dgilland:develop