Handle non-string values in WP_HTML_Tag_Processor

[kraftbj]

Adds handling of non-string values passed to the WP_HTML_Tag_Processor by throwing a _doing_it_wrong and setting it to an empty string to avoid further issues.

Trac ticket: https://core.trac.wordpress.org/ticket/63854

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Props kraftbj, jonsurrell, westonruter.

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[sirreal]

I think ‎WP_HTML_Processor::create_fragment() and WP_HTML_Processor::create_full_parser() are also susceptible. Those can also _doing_it_wrong(), but they're simpler in that they can return null. Do you mind adding those changes here?

[sirreal]

@kraftbj Just checking in, I'd love to get this in 6.9. Are you able to get back to it or would you like help to finish up?

[kraftbj]

Apologies—death in the family after the last comment and it fell off my radar. I'll refresh the branch and make the updates to the other functions now.

[westonruter]

Looks good!

[sirreal]

Thank you! I added tests for the HTML processor.

This is ready to go 👍

[github-actions]

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

SVN changeset: 60887
GitHub commit: 055010b

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