Fix historian null reference

If an MQTT packet was received without custom properties, an exception was thrown. The added optional chaining prevents this error.

historian-uns/src/mqttclient.ts

@@ -150,7 +150,7 @@ export default class MQTTClient {
 
     async on_message(topicString: string, payload: Buffer, packet: mqtt.IPublishPacket) {
         const messageString = payload.toString();
-        if (!packet.properties.userProperties) {
+        if (!packet.properties?.userProperties) {
             logger.error(`⁉ Can't find custom properties for topic ${topicString}! Not writing to Influx.`);
             return
         }