Parent blocks without inner blocks prints empty HTML tags on the front

[carolinan]

Description

I found this issue with both the columns block and the buttons block, and it seemed appropriate to open one issue instead of two.
When you delete all inner button or column blocks, the parent, the wrapper, is still printed on the front of the website.

Of course, this is an edge case that should only happen if a user:

• Is unsure of how to completely delete the block.
• Made some changes and simply forgot to delete the parent.

Step-by-step reproduction instructions

Create a new post.
Add a columns block and a buttons block.
Delete the inner blocks, and save.
View the post on the front, and view the source.
Confirm that the block is present on the page.

Screenshots, screen recording, code snippet

<div class="wp-block-buttons is-layout-flex wp-block-buttons-is-layout-flex"></div>
<div class="wp-block-columns is-layout-flex wp-container-core-columns-is-layout-1 wp-block-columns-is-layout-flex"></div>

Environment info

No response

Please confirm that you have searched existing issues in the repo.

Yes

Please confirm that you have tested with all plugins deactivated except Gutenberg.

Yes

[Mamaduka]

I think this overlaps with #59996.

I see that @Infinite-Null is exploring solutions in #67147. But I think we need a more general solution to check if inner blocks are empty; it could be similar to RichText.isEmpty.

@ellatrix, @youknowriad, has anyone explored a similar solution in the past?

[ellatrix]

For buttons, it should never be the case that only the wrapper remains; we should fix that. If you remove the last button, it should remove the wrapper.

For columns, I'm not sure I agree. If you leave the columns placeholder, it's fine that it leaves markup? I mean, it's obvious that you leave a placeholder in place?

[ellatrix]

For lists and quote, it should be the same fix as buttons. Related: #40979.