Preload paths: avoid fatal error on Widgets page.

[jeherve]

Follow-up to #39256

What and why?

#39256 added some logic using the block_editor_rest_api_preload_paths filter. That works well, but on a fresh installation of WordPress, that filter may sometimes return multidimensional arrays. This is the case on the Widgets page for example, where the filter's array of paths to preload will be as follows:

Array
(
    [0] => Array
        (
            [0] => /wp/v2/media
            [1] => OPTIONS
        )

    [1] => /wp/v2/sidebars?context=edit&per_page=-1
    [2] => /wp/v2/widgets?context=edit&per_page=-1&_embed=about
)

In this situation, when running a version of the Gutenberg plugin including #39256, we end up with this Fatal error when loading the widgets screen:

PHP Fatal error:  Uncaught TypeError: strpos(): Argument #1 ($haystack) must be of type string, array given in /var/www/html/wp-content/plugins/gutenberg/lib/compat/wordpress-6.0/edit-form-blocks.php:44

How?

• This PR solves the issue by ensuring that we're dealing with a string when checking for a matching path.

Testing Instructions

1. Start with a site that runs Gutenberg trunk.
2. Activate a theme that supports sidebars, such as Twenty Twenty.
3. Go to Appearance > Widgets and notice the Fatal error.
4. Apply this patch, and notice that the error disappears.

[jeherve]

cc @jsnajdr who worked on #39256.

[jsnajdr]

Good catch, thanks for fixing! 👍

[jsnajdr]

Hi @jeherve 👋 Can you please rebase your PR branch onto latest trunk? That should be all you need to make the performance and e2e tests pass again. Then we can merge the patch.

[jeherve]

Sure thing; done.