Fix potential fatal error on Interactivity API processing directives

[samueljseay]

This PR is copied from #9641 with the requested changes incorporated.

This may be an edge case for posts created with Classic editor or classic editor block.
_process_directives function within WP_Interactivity_API class will process html and check for closer tags.
if there's a wrong tag like
(in my case) will result in a TypeError:

Fatal error: Uncaught TypeError: count(): Argument #1 ($value) must be of type Countable|array, null given in wp-includes/interactivity-api/class-wp-interactivity-api.php on line 442
by checking the line of code, count() function is applied to get_attribute_names_with_prefix() method from WP_HTML_Tag_Processor which can eventually return null

for newer PHP version (8.0.0 and later) this will throw a fatal error.
probably worth getting an extra check if the funcion is returning a countable variable

Trac ticket: https://core.trac.wordpress.org/ticket/63891

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[github-actions]

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[github-actions]

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

SVN changeset: 61197
GitHub commit: 03171e1

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