extractor: Fix extraction of one file archives

Currently, an empty folder is created when extracting an archive over
Nautilus which contains one file with the same name as the archive.
This is because gnome-autoar always creates the top-level folder, but
the one file archive with the same name should be extracted directly
without creating additional folders. Otherwise the `conflict` signal
is emitted and Nautilus by default skips conflicting files, so only
empty folder remains after extraction. This problem have not been
unfortunately noticed by the `test-one-file-same-name` unit test due to
differences in the `decide-destination` signal handling. Let's do not
create the top-level folder at all as `g_file_make_directory_with_parents`
is used later anyway.

Fixes: https://gitlab.gnome.org/GNOME/gnome-autoar/-/issues/28