fix: preserve falsy non-mapping Markdown frontmatter - #13569

Related Issues

No existing issue; found while reading MarkdownToDocument. This follows the frontmatter support added in #11615. A bounded review of recent open/closed PRs and all frontmatter PR search results found no matching correction.

Proposed Changes

With extract_frontmatter=True, frontmatter containing false, 0, [], or "" is silently removed from the document. These values are not mappings, but or {} converts them to an empty mapping before the existing type check. Other non-mapping values are already kept as content and produce a warning.

Normalize only None to an empty mapping, so falsy non-mappings follow the existing content-preserving path. Empty/null frontmatter and empty mappings retain their behavior. A release note describes the correction.

How did you test it?

• The same nine public run() cases on unchanged source: 4 failed, 5 passed. The four failures show lost frontmatter content.
• Final frontmatter, original Markdown, and MultiFileConverter unit modules: 34 passed, 14 deselected.
• Original Markdown local integration cases: 3 passed, 18 deselected.
• Scoped mypy: no issues in two files. Normal commit hooks, including Ruff, release-note formatting and codespell: passed.
• Full repository tests and remote CI have not been run locally.

Notes for the reviewer

The regression cases are grouped in a dedicated frontmatter test module. Their coverage and assertions are unchanged from the red-before-fix run. An initial attempt to add them to the existing module encountered baseline codespell findings in unrelated encoding byte literals. The existing module is unchanged in this PR and was still executed in full; no hook was bypassed or disabled and no spelling suppression was added.

This issue was selected and the patch implemented, tested, and independently reviewed autonomously by OpenAI Codex agents. The account owner authorized publication. No independent human code review or human test execution is claimed. Agents have not signed a CLA.

Checklist

• Read the contributing guidelines and code of conduct.
• Added regression tests and a release note.
• Used a conventional commit title.
• Ran the relevant unit/integration tests and normal commit hooks.
• Related issue update: not applicable; no separate issue was opened.
• Full repository tests and upstream CI: not yet verified.