fix: fall back from null cmap mappings

[nurikk]

Summary

• treat a decoded null ToUnicode mapping as unusable rather than emitted text
• fall through to the existing glyph-name recovery path
• preserve authoritative Unicode mappings and unresolved-glyph behavior

Why

A PDF character code is a glyph selector, not Unicode U+0000. Partial/broken ToUnicode maps can otherwise place raw null bytes in successful extraction output. This change does not strip nulls or guess a replacement character.

Validation

• regression test demonstrated 3 failures before the production change
• native parser build succeeded
• 32 focused tests passed
• Ruff format/lint
• git diff --check

Scope

This only changes null mappings that have an existing /Differences entry. Broader embedded-font recovery remains the concern of #322.

[github-actions]

✅ DCO Check Passed

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[mergify]

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