Docs: Replace void with null in union return types in class-wpdb.php

[apermo]

Co-Authored-By: xateman

Trac ticket: https://core.trac.wordpress.org/ticket/64703

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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[xateman]

Looks good! 👍

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Props apermo, xate, mukesh27, westonruter.

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[github-actions]

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