Fix test

Author: acogoluegnes

src/main/java/com/rabbitmq/perf/MulticastSet.java

@@ -242,7 +242,9 @@ public void run(boolean announceStartup)
             this.threadingHandler.scheduledExecutorService(
                 "perf-test-heartbeat-sender-", this.params.getHeartbeatSenderThreads());
         factory.setHeartbeatExecutor(heartbeatSenderExecutorService);
-        shutdownService.wrap(heartbeatSenderExecutorService::shutdownNow);
+        if (heartbeatSenderExecutorService != null) {
+          shutdownService.wrap(heartbeatSenderExecutorService::shutdownNow);
+        }
       }
 
       // use a single-threaded executor for the configuration connection