Use first non-empty Nav post as primary fallback for Nav block

[getdave]

Description

In #36724 we added the Page List block as a fallback for when the Nav block has no menu selected.

This PR iterates on that idea by preferring to use the first non-empty Navigation (i.e. wp_navigation Post) as the primary fallback. Only if a suitable Navigation Post is not found do we fallback (again!) to the Page List.

This aims to mirror how the fallback mechanic for wp_nav_menu works currently in Core (as described in the Issue).

Closes #36721

How has this been tested?

Testing is as follows:

### Preparation

• Before testing use Fakerpress Plugin (or similar) to add a load of pages to your site. Some should be nested parent/child.
• Also use the Nav block to create one or two Menus on your site being sure to add some items. Make sure they are saved as Navigation post types (check by going to Appearance -> Navigation).

Testing

• Open Site Editor.
• Remove any existing Nav block.
• Add Nav block from scratch. Don't add any items!
• Save your empty Nav block and publish the post.
• Go to front of site.
• See that the first of your Navigation Menus is rendered by default even though no Menu has been selected.
• Remove all of the items from the Navigation Post that is currently being used as the fallback - this should make it ineligible to be used as a fallback.
• Go to front of site. Check that a different Navigation Post is being used as a fallback.
• Remove all your Navigation Posts - that's right, ditch them all!
• Go to front of site. Check that it falls back to the Page List.

Screenshots

Types of changes

New feature (non-breaking change which adds functionality)

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).

[talldan]

Looks good so far.

[carolinan]

I did a manual test by following the test instructions and did not see any issues. I did not do a code review.

[adamziel]

I left a few suggestions related to naming, but otherwise this looks good to me! If you added a test or two, that would be absolutely stellar. Otherwise, 🚢

[getdave]

I left a few suggestions related to naming, but otherwise this looks good to me! If you added a test or two, that would be absolutely stellar. Otherwise, 🚢

Let's ship and I'll follow up tomorrow with e2e tests. I'll also sort out the Page List block attribute vs context thing.

[bph]

@getdave I did s brief run through this before it was merged but didn't get to add my comments before it go merged.
You had great test instructions! Thank you.

After this sequence:

• Open Site Editor.
• Remove any existing Nav block.
• Add Nav block from scratch. Don't add any items!
• Save your empty Nav block and publish the post.
• Go to front of site.
• See that the first of your Navigation Menus is rendered by default even though no Menu has been selected.

I did see the only menu I had was rendered. Putting myself in the shoes of a site owner or builder. I saw the menu, saw it had the wrong pages in there, so I clicked on the Edit Site link on top of my screen. When the site editor loaded, I didn't know what to do next. My expectation would have been to see the menu now also in the editor, so I can make the changes, I wanted to make.

I do like the default behavior. I just not sure what my expectation tells me about what happens next. Maybe this would work?

Once the default menu is rendered on the frontend, because none was assigned before, that on subsequent Site editor views it could also be rendered there?

Second observation:
After I emptied all menus and trash them I didn't see a navigation rendered on the Front end. That worked. I looked into the Developer Tools and I saw a bunch of navigation html, though

As if there is still an invisible navigation block.
I didn't dive into this yet further, it could come from the theme template, though.

[getdave]

Thanks @bph - appreciate you taking time to test this. I'll double check we can still fallback to Page List and the behaviour around what happens if Page List isn't available.

In terms of the UX we could show a warning on the block that it's currently showing a placeholder on the front end. I'm not confident we could land that in time for 5.9 though given everything else that's still outstanding.

We've also received some feedback / concerns about the fallback situation. I'd like to see how that plays out during the Beta period. It's simple enough that, if we find the majority of folks are not onboard with that, we can revert to simply rendering nothing.

Thanks again 🙇