systemctl: print non-elapsing timers as "n/a" not "(null)"

TimersCalendar={ OnCalendar=1970-01-01 00:00:00 UTC ; next_elapse=(null) }
 ↓
TimersCalendar={ OnCalendar=1970-01-01 00:00:00 UTC ; next_elapse=n/a }

(cherry picked from commit 860f2be0)