fix: 取消订阅

xymanager_service/src/main/java/com/shxy/xymanager_service/mqtt/MqttSubscriberService.java

@@ -66,7 +66,9 @@ public class MqttSubscriberService {
     @PreDestroy
     private void preDestroy() {
         try {
-            mqttClient.unsubscribe(topicList.toArray(new String[0]));
+            if (topicList.size() > 0) {
+                mqttClient.unsubscribe(topicList.toArray(new String[0]));
+            }
             if (mqttClient.isConnected()) {
                 mqttClient.disconnect();
             }