Docs: Update parameter type hints to include null for thumbnail-relat…

[JasonSa19]

As described in ticket 62720, we found several missing types in phpdocs throughout post-thumbnail-template.php.
This PR fixes all missing occurrences of null -types throughout the file.

Trac ticket: https://core.trac.wordpress.org/ticket/62720#ticket

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[github-actions]

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[github-actions]

Hi @JasonSa19! 👋

Thank you for your contribution to WordPress! 💖

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[apermo]

Indent is using tabs instead of spaces, otherwise, it looks good to me.

[apermo]

LGTM

[github-actions]

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[SergeyBiryukov]

Thanks for the PR! Merged in r59548.