main: Register session with GDM on startup

main: Register session with GDM on startup

When no monitor is connected, gnome-shell starts but never emits
`startup-prepared` nor `startup-complete`. As a result, GDM never
detects that the shell has launched and plymouthd remains running,
preventing the system from reaching graphical.target.

Ensure that registerSessionWithGDM() is called even when no monitor
is connected, so GDM can terminate plymouthd and allow the system to
reach graphical.target properly.

Part of https://gitlab.gnome.org/GNOME/gdm/-/merge_requests/285

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/3708>
(cherry picked from commit 32a013304f8c507cb72a5419c7f4b7c553253520)