Scheduled dispatch: Fix handling of missing last/next dispatch times

In some cases

src/timetable_gui.cpp

@@ -196,7 +196,7 @@ static void FillTimetableArrivalDepartureTable(const Vehicle *v, VehicleOrderID
 				predicted_ds.UpdateScheduledDispatchToDate(_state_ticks + sum);
 				StateTicks slot = GetScheduledDispatchTime(predicted_ds, _state_ticks + sum + order->GetTimetabledWait());
 				predicted_ds.ReturnSchedule(ds);
-				if (slot <= -1) return;
+				if (slot == INVALID_STATE_TICKS) return;
 				sum = (slot - _state_ticks).AsTicks();
 				predicted = true;
 				no_offset = true;