Fix fallback not rendering

[draganescu]

What?

Removes a useless check for empty fallback meny blocks.

Why?

#48602 which reverted #47684 failed to remove the code at 8548085#diff-90cb5b64c95087ded3c7bb160e38189c8499b0f14f61beb884c887452449fa12R495

This code now prevents fallnack page list navigation from rendering on the front end.

How?

Remove the left over check for empty blocks. It only makes sense in the if branch before, where $maybe_fallback may be empty because of block_core_navigation_filter_out_empty_blocks.

Testing Instructions

1. Make sure to have one page
2. Delete all navigation block menus (from wp-admin/edit.php?post_type=wp_navigation)
3. Install and activate the theme Lettre
4. Check that the front end renders a page list

[github-actions]

Flaky tests detected in 7ed8a81.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/4606794317
📝 Reported issues:

• [Flaky Test] doesn't throw a block error when clicking on a link #47747 in /test/e2e/specs/site-editor/template-part.spec.js
• [Flaky Test] prevent the removal of locked column block from the column count change UI #49534 in /test/e2e/specs/editor/blocks/columns.spec.js
• [Flaky Test] should insert elements from multiple completers in a single block #47916 in /test/e2e/specs/editor/various/autocomplete-and-mentions.spec.js

[MaggieCabrera]

This tests well and looking at the reverted PR the code makes sense to me

[ryanwelcher]

I just cherry-picked this PR to the release/15.5 branch to get it included in the next release: 3180818

[getdave]

Is there a way to cover this with a test?