Real-time collaboration: Remove ghost awareness state explicitly when refreshing

[alecgeatches]

Backport of PHP changes from WordPress/gutenberg#75883. Changes the real-time collaboration polling server endpoint to accept null for awareness values, which is used to explicitly disconnect users leaving the page and remove their awareness data more quickly.

Trac ticket: https://core.trac.wordpress.org/ticket/64622

Use of AI Tools

Claude for the original PR.

[github-actions]

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[github-actions]

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[github-actions]

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[github-actions]

A commit was made that fixes the Trac ticket referenced in the description of this pull request.

SVN changeset: 61746
GitHub commit: 91f6c42

This PR will be closed, but please confirm the accuracy of this and reopen if there is more work to be done.