Adapts Dir_file_system::open_composite_dirs in a way that it returns "success"
when the leaf node of the path is an empty directory but "lookup failed", as
usual, if one of the other directories on the way to the leaf node is empty.
I couldn't find a technical reason why we used to return "lookup failed" when
only the leaf node was empty.
The commit also adds a test for en empty root directory and empty
sub-directories to the fs_query run script.
Fixes#4198
16c4aacf34