Allow filtering of Nav block fallback

[getdave]

Description

In #36740 (comment) we introduced a specific fallback mechanic for the Nav block front-of-site rendering for the case where a Menu was not specifically assigned to the block.

This aimed to follow the existing code patterns set by wp_nav_menu.

That PR didn't handle allowing developers to filter the fallback output of the block. This PR enables that via the introduction of a block-specific filter block_core_navigation_render_fallback.

This is important because traditionally Theme developers have had the option to opt-out of the wp_nav_menu's fallback mechanic via the fallback_cb argument that you can pass to that function.

This PR replicates that. Developers can now do this to opt out of rendering a fallback:

add_filter('block_core_navigation_render_fallback', '__return_false');

Alternatively they can return their own list of blocks to act as a fallback with the caveat that these blocks must be valid as children of the core/navigation block.

If folks are in favour of merging this PR then I'll document the filter.

Note this overlaps with #36849. If/when that is merged then we'll consolidate the "empty" checking mechanics.

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 add the filter - I did this in a test mu-plugin, but you could simply add to your Theme's functions.php:

add_filter('block_core_navigation_render_fallback', '__return_false');

• Visit front of site. You should see that no navigation block is rendered at all - you've opted out of the the fallback experience.

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).

[getdave]

cc @fklein-lu also for review

[getdave]

Suggested change

[getdave]

Let's leave this open overnight (UTC) to see if there's further feedback. If I don't receive any then I'll merge this based on @adamziel's 👍

[fklein-lu]

Cheers for the quick follow up 👍

[getdave]

Ok as there was no further feedback I will merge this.