Fix `EditorInterface.get_selected_paths()` working incorrectly when FileSystemDock is in split mode

[fstxz]

Fixes #88228

[AThousandShips]

Will try and test this tomorrow

[fstxz]

I wonder if folders that are selected in the tree should also be included when in split mode? Otherwise you can't select multiple folders that are in different places without changing the display mode to tree only.

[KoBeWi]

In split mode if you have a folder selected, but none of the files, the returned list is empty. If no file is selected, maybe the selected directory should be returned at least?

[fstxz]

@KoBeWi I updated the PR, I hope this is what you meant.

[fstxz]

Seems like it's possible to multi-select in tree in split view

It's also possible to deselect everything with ctrl, this is why I chose to return ony the current directory.

[akien-mga]

Thanks! And congrats for your first merged Godot contribution :tada:

[akien-mga]

Cherry-picked for 4.3.1.