added test if backup directory exists before old files are tried to be deleted

contrib/backup/functions

@@ -12,7 +12,7 @@ function get_backup_date () {
 }
 
 function delete_old_backups () {
-    find ${BACKUP_DIR}/*_zammad_*.gz -type f -mtime +${HOLD_DAYS} -exec rm -r \;
+    test -d ${BACKUP_DIR} && find ${BACKUP_DIR}/*_zammad_*.gz -type f -mtime +${HOLD_DAYS} -exec rm -r \;
 }
 
 function get_db_credentials () {