fix(go): skip empty frames in realtime snapshot

go/vehicle-gateway/internal/realtime/snapshot_writer.go

@@ -35,6 +35,9 @@ func (w *SnapshotWriter) Update(ctx context.Context, env envelope.FrameEnvelope)
 	if vehicleKey == "" || strings.HasSuffix(vehicleKey, ":unknown") {
 		return nil
 	}
+	if !hasRealtimePayload(env) {
+		return nil
+	}
 	fieldsJSON, err := marshalObject(env.Fields)
 	if err != nil {
 		return err
@@ -82,6 +85,16 @@ func nullableTime(ms int64) any {
 	return time.UnixMilli(ms)
 }
 
+func hasRealtimePayload(env envelope.FrameEnvelope) bool {
+	if len(env.Fields) > 0 {
+		return true
+	}
+	if _, ok := env.Parsed["data_units"]; ok {
+		return true
+	}
+	return false
+}
+
 const realtimeSnapshotTableSQL = `CREATE TABLE IF NOT EXISTS vehicle_realtime_snapshot (
   id BIGINT PRIMARY KEY AUTO_INCREMENT,
   protocol VARCHAR(32) NOT NULL,