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)
Status Job ID Name Coverage
  Docker Image
passed #3721610
debian-testing

00:00:19

 
  Build And Test
passed #3721611
autotools

00:05:50

passed #3721612
meson

00:01:52

passed #3721613
meson-noglamor

00:01:49