Get broker version only if connection is open

(cherry picked from commit 3ecf0c5)

Author: acogoluegnes

src/test/java/com/rabbitmq/client/test/BrokerTestCase.java

@@ -68,7 +68,9 @@ public void setUp(TestInfo testInfo) throws IOException, TimeoutException {
         Assumptions.assumeTrue(shouldRun());
         this.testInfo = testInfo;
         openConnection();
-        this.brokerVersion = currentVersion(this.connection.getServerProperties().get("version").toString());
+        if (this.connection != null) {
+            this.brokerVersion = currentVersion(this.connection.getServerProperties().get("version").toString());
+        }
         openChannel();
 
         createResources();