[broker-25] fix for code review

Author: JavaSaBr

src/main/java/com/ss/mqtt/broker/model/SubscribeTopicFilter.java

@@ -51,7 +51,7 @@ public SubscribeTopicFilter(@NotNull TopicFilter topicFilter, @NotNull QoS qos)
     }
 
     @Override
-    public String toString() {
+    public @NotNull String toString() {
         return "SubscribeTopicFilter(" + "topicFilter=" + topicFilter.getRawTopic() + ", qos=" + qos + ", retainHandling=" +
             retainHandling + ", noLocal=" + noLocal + ", retainAsPublished=" + retainAsPublished + ')';
     }

src/test/groovy/com/ss/mqtt/broker/test/integration/ConnectSubscribePublishTest.groovy

@@ -15,7 +15,7 @@ class ConnectSubscribePublishTest extends IntegrationSpecification {
         given:
             def received = new AtomicReference<Mqtt5Publish>()
             def subscriber = buildClient()
-            def subscriberId = subscriber.getConfig().clientIdentifier.get()toString()
+            def subscriberId = subscriber.getConfig().clientIdentifier.get().toString()
             def publisher = buildClient()
         when:
             subscriber.connect().join()
@@ -48,7 +48,7 @@ class ConnectSubscribePublishTest extends IntegrationSpecification {
         given:
             def received = new AtomicReference<Mqtt5Publish>()
             def subscriber = buildClient()
-            def subscriberId = subscriber.getConfig().clientIdentifier.get()toString()
+            def subscriberId = subscriber.getConfig().clientIdentifier.get().toString()
             def publisher = buildClient()
         when:
 
@@ -82,7 +82,7 @@ class ConnectSubscribePublishTest extends IntegrationSpecification {
         given:
             def received = new AtomicReference<Mqtt5Publish>()
             def subscriber = buildClient()
-            def subscriberId = subscriber.getConfig().clientIdentifier.get()toString()
+            def subscriberId = subscriber.getConfig().clientIdentifier.get().toString()
             def publisher = buildClient()
         when: