FIX: fixed the default return of the tcp classes

src/DirectCommunicationClient.cpp

@@ -54,7 +54,7 @@ namespace DirectCommunication
             receivedMessages_.get(msg);
             return msg;
         }
-        return nullptr;
+        return "NULL";
     }
 
     void DirectCommunicationClient::workerFunc_()

src/DirectCommunicationServer.cpp

@@ -55,7 +55,7 @@ namespace DirectCommunication
             receivedMessages_.get(msg);
             return msg;
         }
-        return nullptr;
+        return "NULL";
     }
 
     int DirectCommunicationServer::countClients()