No drop indicator appears when dragging a file into an empty post or block list

[talldan]

Description

Related #32880, #42365, #42722

Dropping a file does work, but there's no indicator to show that it will work.

This seems to happen because there are no blocks in the post. The 'Type / to choose a block' text is part of a React component called the DefaultBlockAppender, so the usual block drop indicators don't work (and they wouldn't be the right thing to show anyway).

There's also a similar situation when trying to drag a block or a file into an empty group block, so it may be worth coming up with one solution for empty block lists.

Step-by-step reproduction instructions

Empty post

1. Create a new post
2. Try dragging a file into the post

Expected: There's some indication on the screen that dropping the file will work

Empty group block

1. Add a group block
2. Try dragging another block or image into the group

Expected: There's some indication on the screen that dropping the block will work

Screenshots, screen recording, code snippet

No response

Environment info

• Brave / Mac OS

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