Fix Nav block fallback DB query to match on full block grammar start tag

[getdave]

Description

Currently when seeking fallback Navigation Menus to use as the front-of-site fallback for the block rendering the Nav block looks for opening <!-- tags to fetch the first post that has blocks.

However this is vulnerable to error because the post might contain a non-block HTML comment which would then be treated as valid.

Whilst we are protected by the code which filters out the resulting null blocks that would be returned by the block parser we can safe guard against this in advance by searching for the full block comment <!-- wp:.

I checked and this comment is used by all blocks, even those which aren't core blocks. Here's an example using Kadence blocks:

How has this been tested?

• Delete all Navigation Menu posts from your site - http://localhost:8888/wp-admin/edit.php?post_type=wp_navigation.
• Add several Pages to your site.
• Add a Nav block to your site. Add some items.
• Save the Site Editor and thus the Nav Menu.
• Reload the Site Editor.
• Go to the Nav block and delete it. Then re-add an empty Nav block and save the Editor.
• Now go into your database and modify the Navigation Menu post to contain an HTML comment that is not block grammar. Eg:

<!-- Some other random HTML comment -->And some stuff here!

• Visit front of site. You should see the core/page-list block used to render the default fallback experience.
• You should not see the Navigation used because it is invalid as blocks.

Screenshots

Types of changes

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).