dtcm: prevent to use NULL as a Tick type

Tick (aka time_t) shall be an integer type, not a pointer.
Change to use `0` instead of `NULL`.

cde/programs/dtcm/server/reutil.c

@@ -230,7 +230,7 @@ WeekNumberToDay(
          * month.
          */
         if (date_tm->tm_mon != initial_month_number)
-                return ((Tick)NULL);
+                return (Tick)0;
 
         return (_date);
 }