修改Mqtt的bug

seatunnel-datasource/seatunnel-datasource-plugins/datasource-mqtt/src/main/java/org/apache/seatunnel/datasource/plugin/mqtt/InitCallback.java

@@ -6,6 +6,7 @@ import org.eclipse.paho.client.mqttv3.MqttMessage;
 
 import lombok.extern.slf4j.Slf4j;
 
+import java.util.ArrayList;
 import java.util.List;
 
 /**
@@ -17,7 +18,7 @@ import java.util.List;
 @Slf4j
 public class InitCallback implements MqttCallback {
 
-    private List<MqttMessage> mqttMessageList;
+    private List<MqttMessage> mqttMessageList=new ArrayList<>();
 
     public List<MqttMessage> getMqttMessageList() {
         return mqttMessageList;