tests: Add nullable check to wl_message_count_arrays

Array argument symbols in a wl_message may be nullable, but the test for
wl_message_count_arrays did not test this. Add one more wl_message with
nullable array arguments.

Signed-off-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>