At the moment the timed login feature is implemented in the user list.
If there's no user list, we don't show the indicator anywhere and
don't proceed with timed login.
This commit allows timed login to work when the user list is disabled.
It accomplishes this by putting the timed login indicator on the
auth prompt in that scenario.
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/1809>