desktop-shell: do not lower_fullscreen_layer(s, NULL)

In activate, do not call lower_fullscreen_layer() at all if the output
is NULL. It should not do anything in that case, per the existing
comment.

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

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>