We treat an empty wall-message equal to a NULL wall-message since:
commit
5744f59a3ee883ef3a78214bd5236157acdc35ba
Author: Lennart Poettering <lennart@poettering.net>
Date: Fri Sep 4 10:34:47 2015 +0200
logind: treat an empty wall message like a NULL one
Fix the shutdown scheduler to not deref a NULL pointer, but properly
check for an empty wall-message.
Fixes: #1120