Enhancement: Display a Button Appender when Buttons block is empty

[aurooba]

Based on a Twitter conversation with @heathergray (closes #38338), it looks like the lack of a visual cue that the Buttons block is still in the editor, seems to cause confusion for more than one person. Although, upon refreshing the Buttons block auto populates with an empty Button block, it can be frustrating not realizing that deleting the Button block doesn't mean you've deleted the Buttons parent block.

Description

To solve this issue, I've changed the Buttons block just slightly, to show the Button Appender when the block is empty, making it easy to see that the Buttons block is still in the editor, you can easily select it to delete/add/etc.

Testing Instructions

1. Open up the Block Editor
2. Add a Buttons block
3. Delete all Button blocks
4. See the appender appear when the Buttons block is empty.

Screenshots

Screen.Recording.2022-01-28.at.10.37.30.AM.mov

Types of changes

When empty, the Buttons block displays the Button Appender to indicate its existence.

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).
• I've updated related schemas if appropriate.