[broker-11] fix for code review

Author: JavaSaBr

src/main/java/com/ss/mqtt/broker/network/client/impl/AbstractMqttClientReleaseHandler.java

@@ -49,10 +49,6 @@ public abstract class AbstractMqttClientReleaseHandler<T extends AbstractMqttCli
             asyncActions = clientIdRegistry.unregister(clientId);
         }
 
-        if (asyncActions != null) {
-            return asyncActions;
-        }
-
-        return Mono.empty();
+        return asyncActions;
     }
 }