desktop-shell: survive NULL output in shell_configure_fullscreen()

Running 'weston-simple-egl -f -b' (fullscreen, unthrottled) caused a
crash in shell_ensure_fullscreen_black_view() due to
shsurf->fullscreen_output being NULL. Also shell_configure_fullscreen()
could crash on that condition.

Fix shell_configure_fullscreen() to bail out with minimal work if there
is no fullscreen_output.

It is unclear if anything will cause a reconfiguration when an output is
plugged in.

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>