fix(#189): apply(bool) if repos are not empty

[h1alexbel]

ref #189

• feat(TypeError: object of type 'NoneType' has no len() #189): test case
• feat(TypeError: object of type 'NoneType' has no len() #189): remove NaN first

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PR-Codex overview

This PR focuses on enhancing the handling of repositories with null headings in the extraction process, updating the test suite, and making adjustments to the build configuration.

Detailed summary

• Added a new CSV file to-extract-null-headings.csv for testing.
• Updated the build.yml to use macos-13.
• Introduced a new test test_skips_repo_with_null_headings in test_extract.py.
• Adjusted the extraction logic in extract.py to drop rows with null headings before further processing.
• Ensured that mcw is only calculated if headings exists in the DataFrame.

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[h1alexbel]

@rultor merge

[rultor]

@rultor merge

@h1alexbel OK, I'll try to merge now. You can check the progress of the merge here.

[rultor]

@rultor merge

@h1alexbel Done! FYI, the full log is here (took me 9min).