desktop-shell: Return NULL when no outputs are present
desktop-shell: Return NULL when no outputs are present

Currently, get_default_output returns a first member
of the linked list, which can never be NULL.

This is problematic, as the function would return a
dangling pointer and NULL pointer checks wouldn't
work where needed and some of the invalid members
would get accessed that way, resulting in a crash.

Reviewed-by: Quentin Glidic <241385a0e41ea713327e189bce1062decf7c1eb5@sardemff7.net>
Reviewed-by: Pekka Paalanen <0f6ae7001e34c37b02fb2c1c2573ddf8f442136a@collabora.co.uk>
Signed-off-by: Armin Krezović <31ec4b88003c779f400d1506dcff569d5faae2fb@gmail.com>

krezovic/weston,sir-murray/weston,sir-murray/weston,krezovic/weston