Change client id validation range in codec-mqtt

Motivation:

In MQTT 3.1 specification, "The Client Identifier (Client ID) is between
1 and 23 characters long, and uniquely identifies the client to the
server". But, current client id validation length is 0~23. It must be
1~23. The empty string is invalid client id in MQTT 3.1

Modifications:

Change isValidClientId method. Add MIN_CLIENT_ID_LENGTH.

Result:

The validation check for client id length is between 1 and 23.

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

@@ -21,6 +21,7 @@
 final class MqttCodecUtil {
 
     private static final char[] TOPIC_WILDCARDS = {'#', '+'};
+    private static final int MIN_CLIENT_ID_LENGTH = 1;
     private static final int MAX_CLIENT_ID_LENGTH = 23;
 
     static boolean isValidPublishTopicName(String topicName) {
@@ -38,7 +39,8 @@ static boolean isValidMessageId(int messageId) {
     }
 
     static boolean isValidClientId(String clientId) {
-        return clientId != null && clientId.length() <= MAX_CLIENT_ID_LENGTH;
+        return clientId != null && clientId.length() >= MIN_CLIENT_ID_LENGTH &&
+            clientId.length() <= MAX_CLIENT_ID_LENGTH;
     }
 
     static MqttFixedHeader validateFixedHeader(MqttFixedHeader mqttFixedHeader) {