xwayland: Handle NULL xwl_seat in xwl_seat_can_emulate_pointer_warp

This can happen e.g. with weston's headless backend.

Reviewed-by: Olivier Fourdan <ofourdan@redhat.com>
4 jobs for xwl_seat_can_emulate_pointer_warp-NULL in 6 minutes and 10 seconds (queued for 1 second)