Docs: Replace void with null in union return types in Customize

[apermo]

Co-Authored-By: xateman

Trac ticket: https://core.trac.wordpress.org/ticket/64701

Use of AI Tools

Used AI for research, documentation and for the replacements. Everything was reviewed by myself and @xateman before opening this PR.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

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[xateman]

Looks good 👍

[github-actions]

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[westonruter]

LGTM

[github-actions]

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

SVN changeset: 61767
GitHub commit: ee837f5

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