desktop-shell: handle NULL output in center_on_output()

This is a tentative crash fix for a case where there are no enabled
weston_outputs at all.

If no output is given, just put the surface at 0,0. At least it should
become mostly visible if an output is plugged in, if not centered.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>
Signed-off-by: Fabien Lahoudere <fabien.lahoudere@collabora.com>
Reviewed-by: Ian Ray <ian.ray@ge.com>