serve: format proper StopTimeEvents 🐛

The previous format (empty arrival and/or departure object) causes errors in OpenTripPlanner.

The GTFS-Realtime spec is very clear about this:

> Either delay or time must be provided within a StopTimeEvent - both fields cannot be empty.

related: opentripplanner/OpenTripPlanner#5050

lib/gtfs-rt-writer.js

@@ -29,21 +29,30 @@ const {STOPPED_AT, IN_TRANSIT_TO} = VehiclePosition.VehicleStopStatus
 
 const formatStopTimeUpdate = (st) => {
 	const s = st.stop
+
 	const arr = st.arrival || st.prognosedArrival || st.plannedArrival
+	const arrTime = arr ? formatWhen(arr) : null
+	const arrDelay = 'number' === typeof st.arrivalDelay ? st.arrivalDelay : null
+	const arrival = arrTime !== null || arrDelay !== null ? {
+		time: arrTime,
+		delay: arrDelay,
+		// todo: uncertainty
+	} : null
+
 	const dep = st.departure || st.prognosedDeparture || st.plannedDeparture
+	const depTime = dep ? formatWhen(dep) : null
+	const depDelay = 'number' === typeof st.departureDelay ? st.departureDelay : null
+	const departure = depTime !== null || depDelay !== null ? {
+		time: depTime,
+		delay: depDelay,
+		// todo: uncertainty
+	} : null
+
 	return {
 		stop_id: s && (s.ids && s.ids.gtfs || s.id) || null,
 		stop_sequence: 'number' === typeof st.stopoverIndex ? st.stopoverIndex : null,
-		arrival: {
-			time: arr ? formatWhen(arr) : null,
-			delay: 'number' === typeof st.arrivalDelay ? st.arrivalDelay : null
-			// todo: uncertainty
-		},
-		departure: {
-			time: dep ? formatWhen(dep) : null,
-			delay: 'number' === typeof st.departureDelay ? st.departureDelay : null
-			// todo: uncertainty
-		},
+		arrival,
+		departure,
 		schedule_relationship: st.cancelled ? SKIPPED : SCHEDULED
 	}
 }