desktop-shell: handle NULL output in get_output_work_area()

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

Let get_output_work_area() return a zero area if the given output is
NULL. If there is no output, there is no area. Unfortunately we cannot
return "no position" but have to use 0,0 instead.

In send_configure_for_surface(), this causes a maximized surface to
receive width=0 and height=0 in the configure event, which means the
client is free to choose the size. There is no correct size to send for
maximizing for no output.

In constrain_position(), this has no effect. The interactive move of a
surface is restricted to not go below the panel, so even if a user
managed to move a surface without an output, it just prevents the
surface moving beyond y=0.

In weston_view_set_initial_position(), get_output_work_area() will not
be called with NULL output anyway.

In set_maximized_position(), this makes it behave as if the output was
at 0,0 which is the default position of the first output.

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>