Template part has empty section in toolbar

[carolinan]

Description

With current Gutenberg trunk active, there is an empty section / panel in the toolbar of template parts,
between the buttons that move the block up and down and the Edit button.

Step-by-step reproduction instructions

Activate a block theme, for example, Twenty Twenty-Four.
Go to Appearance > Editor. Click on the preview to edit the current template.
Select an existing template part or add a new.
Confirm that there is an empty section in the toolbar.

Screenshots, screen recording, code snippet

Actual: there is an extra, empty section in the toolbar.

Expected: no empty panel in the toolbar.

Environment info

Only tested on WP 6.4 RC 2, with and without Gutenberg trunk.

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

Yes

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

No

[t-hamano]

My guess is that this problem occurs when there are no fills in the BlockControls' slots at all. For example, you can see this problem in a Group block or Columns block within a Group block with "
Inner blocks use content width" turned off.

[Mamaduka]

I think I know what caused this regression. I will look into it later today.