Fix missing return value in force_filtered_html_on_import footnotes filter

[aduth]

What?

This PR adds back a missing return value of the force_filtered_html_on_import filter in the _gutenberg_footnotes_force_filtered_html_on_import_filter callback. Previously it did not return a value on one of its branches.

Why?

A filter callback must always return the value it receives. In our function, when the core function _wp_filter_post_meta_footnotes() exists, the code did return;. This returns null instead of the original value.

How?

The fix is very small. On the short-circuit branch we now return the input value instead of nothing:

if ( function_exists( '_wp_filter_post_meta_footnotes' ) ) {
	return $arg; // Before: return;
}

Testing Instructions

Verify PHP tests pass.

Use of AI Tools

Changes applied manually.

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: aduth <aduth@git.wordpress.org>
Co-authored-by: aaronjorbin <jorbin@git.wordpress.org>
Co-authored-by: tyxla <tyxla@git.wordpress.org>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[aduth]

I just cherry-picked this PR to the release/23.7 branch to get it included in the next release: f54adf3