Fix rule filtering in the document walker

[mclem]

🤔 What's changed?

Rules are now filtered when their all their children (background and scenarios) are themselves filtered.

⚡️ What's your motivation?

Fix for https://github.com/cucumber/react-components/issues/136

🏷️ What kind of change is this?

• :bug: Bug fix (non-breaking change which fixes a defect)

♻️ Anything particular you want feedback on?

I have no experience in TypeScript so I'm not sure about the fix. It seems that the bug was about a difference in handling of undefined vs null. Please feel free to modify anything as required.

📋 Checklist:

• [x] I agree to respect and uphold the Cucumber Community Code of Conduct
• [x] I've changed the behaviour of the code
  • [x] I have added/updated tests to cover my changes.
• [ ] My change requires a change to the documentation.
  • [ ] I have updated the documentation accordingly.
• [ ] Users should know about my change
  • [ ] I have added an entry to the "Unreleased" section of the CHANGELOG, linking to this pull request.

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[luke-hill]

@mpkorstanje / @Dbuggerx can this now be closed?

[mpkorstanje]

Superseded by https://github.com/cucumber/gherkin-utils/pull/48