Render empty Nav block if no fallback block can be utilised

[getdave]

Description

In #36740 (comment) we learnt that if a page list block isn't registered then the block will still render wrapping markup even though it is empty.

This PR fixes things by first checking whether the core/page-list fallback is registered. If it is not and the fallback mechanic attempts to use it then the block will not render.

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. Remove all items from the Nav block.
• Save the Site Editor.
• Visit front of site. You should see the core/page-list block used to render the default fallback experience.
• Now...unregister the core/page-list block. I used a mu-plugin on my test site. Simply copy the following into a file and save with any name you like. Then zip it up and upload it to your WP site as a Plugin. Then activate the Plugin.

function my_plugin_deny_list_blocks() {
	$registry = WP_Block_Type_Registry::get_instance();

	if ( $registry->is_registered( 'core/page-list' ) ) {
		unregister_block_type( 'core/page-list' );
	}
}
add_action( 'wp_loaded', 'my_plugin_deny_list_blocks' );

• Double check you can no longer access the Page List block in the editors.
• Go to the front of your site - check that the Nav block is no longer being rendered at all. No markup or anything.

Screenshots

Types of changes

Bug fix (non-breaking change which fixes an issue)

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).