login: fix NULL-deref on wall_message

login: fix NULL-deref on wall_message

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