Add default block in MqttVersion (#11226)

Motivation:

Fix switch case fall through, add default block in MqttVersion

Modification:

Fix switch case fall through, add default block in MqttVersion

Result:

Code cleanup


Signed-off-by: xingrufei <xingrufei@sogou-inc.com>

Co-authored-by: xingrufei <xingrufei@sogou-inc.com>

codec-mqtt/src/main/java/io/netty/handler/codec/mqtt/MqttVersion.java

@@ -60,6 +60,8 @@ public enum MqttVersion {
         case 5:
             mv = MQTT_5;
             break;
+        default:
+            break;
         }
         if (mv == null) {
             throw new MqttUnacceptableProtocolVersionException(protocolName + "is unknown protocol name");