Fails to start if all of xdg-config is made available

[MithicSpirit]

If xdg-config is shared with steam, then the flatpak fails to start with the
error below. The issue seems to be that _get_host_xdg_mounts splits the path
into their components and, if the first component is xdg-config (or -data or
-cache), then it re-joins all subsequent components, since this only has one
component, there are no other components, to join, which is an error since
os.path.join expects at least one component.

INFO:root:https://github.com/flathub/com.valvesoftware.Steam/wiki
ERROR:root:~/.steam/root isn't a symlink to an existing directory, cannot determine current prefix
INFO:root:Will set XDG dirs prefix to /home/mithic/.var/app/com.valvesoftware.Steam
Traceback (most recent call last):
  File "/app/bin/steam", line 8, in <module>
    sys.exit(main())
             ^^^^^^
  File "/app/lib/python3.11/site-packages/steam_wrapper.py", line 486, in main
    should_restart = migrate_config(current_info, xdg_dirs_prefix)
                     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/lib/python3.11/site-packages/steam_wrapper.py", line 316, in migrate_config
    ignore = _get_host_xdg_mounts("xdg-config", flatpak_info)
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/lib/python3.11/site-packages/steam_wrapper.py", line 305, in _get_host_xdg_mounts
    dirs.add(os.path.join(*path_seq[1:]))
             ^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: join() missing 1 required positional argument: 'a'