MQTTv3.1 supports a zero-byte clientId during authentication (#900)

Thank's for the contribution, you are right.

Co-authored-by: liujialiang <jialiang.liu@coolkit.cn>

Author: l2063610646

broker/src/main/java/io/moquette/broker/MQTTConnection.java

@@ -210,7 +210,7 @@ PostOffice.RouteResult processConnect(MqttConnectMessage msg) {
         final boolean cleanSession = msg.variableHeader().isCleanSession();
         final boolean serverGeneratedClientId;
         if (clientId == null || clientId.isEmpty()) {
-            if (isNotProtocolVersion(msg, MqttVersion.MQTT_5)) {
+            if (isNotProtocolVersion(msg, MqttVersion.MQTT_5) && isNotProtocolVersion(msg, MqttVersion.MQTT_3_1_1)) {
                 if (!brokerConfig.isAllowZeroByteClientId()) {
                     LOG.info("Broker doesn't permit MQTT empty client ID. Username: {}", username);
                     abortConnection(CONNECTION_REFUSED_IDENTIFIER_REJECTED);