refactor(rocketmq-spring-boot): 优化 RocketMQMessageHandler 默认实现

- 为 rocketMQMessageHandler 添加默认实现，提高代码健壮性

Author: k631583871

rocketmq-spring-boot/src/main/java/org/apache/rocketmq/spring/support/DefaultRocketMQListenerContainer.java

@@ -140,7 +140,7 @@ public class DefaultRocketMQListenerContainer implements InitializingBean,
 
     private String instanceName;
 
-    private RocketMQMessageHandler rocketMQMessageHandler;
+    private RocketMQMessageHandler rocketMQMessageHandler = (message, chain) -> chain.doHandler(message);
 
     public long getSuspendCurrentQueueTimeMillis() {
         return suspendCurrentQueueTimeMillis;