"Move to" not showing when there's only one block at top level

[tellthemachines]

Description

"Move to" in the block settings dropdown only shows if there is more than one top-level block. For example, if all the post content is contained inside a Columns block, it is not possible to use the "Move to" feature on any of its inner blocks.

Step-by-step reproduction instructions

1. In a post, add a Group block, and add multiple blocks inside it.
2. Verify that "Move to" doesn't show in the block settings dropdown for any of those blocks inside the Group block.
3. Add another block outside of the Group block.
4. Now "Move to" should show for all blocks.

Screenshots, screen recording, code snippet

No response

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]

Related #33158.