fix process-zhanna error handling

plugins/zhanna.lisp

@@ -84,7 +84,7 @@
 (defsetting *zhanna-subscriptions* nil "chat-ids of schedule changes receivers")
 (defvar *zhanna-last-schedule* nil "Schedule from last cron run")
 (defcron process-zhanna ()
-  (let ((schedule (zhanna-get-schedule 1 2 3 4 5 6 0)))
+  (alexandria:when-let ((schedule (zhanna-get-schedule 1 2 3 4 5 6 0)))
     (when *zhanna-last-schedule*
       (handler-case
           (let* ((day-changes (loop for dow in +zhanna-dows+