Add logic to prevent ungroup option when there are no inner blocks

[talldan]

Description

Prevents the ungroup option being displayed in the options menu when a group block has no inner blocks.

How has this been tested?

Added an e2e test

Manual testing:

1. Add a group block
2. Open the block more menu

Actual behaviour (master)
The ungroup option is visible, and when clicked does nothing

Expected behaviour ( this branch )
The ungroup option is not visible.

Types of changes

Bug fix (non-breaking change which fixes an issue)

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.

[gziolo]

For reference, this is how it works in master without this patch:

[jorgefilipecosta]

Works well on my tests and the change looks good 👍