Handle innerContent too when removing innerBlocks

[draganescu]

What?

Fixes #46371
Alternative to #46374

Why?

In #46374 we're adding one item but we're appeasing the system to not throw a fatal error, instead of making it behave as we expect. In this PR the behavior is as expected.

How?

In our filtering function block_core_navigation_filter_out_invalid_blocks if one of the innerBlocks is not in the allowed list we set the parent's innerBlocks to []. This triggers a fatal error in WP_Block because it has a conflict in the parsed block between the innerBlocks being [] and innerContent still retaining the old shape, e.g. for a social links block with a filtered social link

["innerContent"]=>
  array(3) {
    [0]=>string(35) "<ul class="wp-block-social-links">"
    [1]=>NULL
    [2]=>string(6) "</ul>"
  }

because of this we get into a spiral that ends in the fatal error.

This PR sets innerContent to be a string by imploding the array above. If there are no inner blocks we only care about the empty parent block.

Caveat

Manually unsetting innerblocks like we do in the navigation block (set to []) is "off label" use of how blocks expect to be dealt with. We either find a nice way to remove a block from the rendering process, or we will probably hit other problems if the navigation block becomes even more complex.

The case we seem to prevent at render time should not exist if the block was created in the editor, in other words allowed blocks in the editor should not be repeated in the rendering function b/c no unallowed block should be there. Generally we treat content input manually e.g. via code view "as is". The exception here is that in #46279 a possibility for infinite loop was identified - most likely via manual editing.

Conclusion

My opinion is that we should revert #46279 and not merge neither this or #46374. We should maybe add a safeguard to avoid infinite nesting my adding a max nesting a la react, silently error and not render anything.

Testing Instructions

1. Add a navigation block
2. Add some navigation links
3. Add a social links block
4. Add a social link
5. Save and publish
6. Check the front end
7. The social links block should not render but there should be no PHP error

Testing Instructions for Keyboard

N/A

Screenshots or screencast

N/A

[ajlende]

This fixes the page from erroring out completely, but we also need the allowed blocks from #46374

I'll open a PR shortly for reverting the recursive change. I think it's important for the quick fix to go in, and then we can continue discussion on the new PR.

[getdave]

I think it's important for the quick fix to go in, and then we can continue discussion on the new PR.

Thank you for being pragmattic in the approach to merging to fix the initial problem 🙇