This fixes #53869.

Creating a new file from the project panel background menu failed when a
single local project was empty and its root was hidden. In that state
there are no visible entries to seed `expanded_dir_ids`, so the action
returned early before opening the filename editor.

This initializes that state lazily from the root entry when creating a
new item, and adds a regression test for the empty hidden-root path.

Release Notes:

- Fixed creating a new file from the project panel context menu in empty
local projects