fix(Collections): handle falsy keys properly

[reidbarber]

Closes #9991

In several places we were treating a key of 0 or "" as missing due to checking truthiness instead of nullishness.

✅ Pull Request Checklist:

• Included link to corresponding React Spectrum GitHub Issue.
• Added/updated unit tests and storybook for this change (for new code or code which already has tests).
• Filled out test instructions.
• Updated documentation (if it already exists for this component).
• Looked at the Accessibility Practices for this feature - Aria Practices

📝 Test Instructions:

Unit tests should cover.

🧢 Your Project:

[rspbot]

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[LFDanLu]

LGTM, but mind adding some tests around rows with falsy keys attached to them so we don't regress in the future?

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[snowystinger]

That was a lot of places, we really need a lint rule or ts plugin to catch those more