Exception Handling for Null Pointer Exception in BrokerService (apach…

…e#3108)

### Motivation
In a unit test under pulsar-broker, I have found that some NullPointerExceptions were thrown, although the tests had passed. I decided it will be best if we minimized the number of such exceptions as they are usually not intentionally thrown.

### Result
The NullPointerException will no longer occur.

pulsar-broker/src/main/java/org/apache/pulsar/broker/service/BrokerService.java

@@ -427,7 +427,7 @@ public void close() throws IOException {
     public void unloadNamespaceBundlesGracefully() {
         try {
             // make broker-node unavailable from the cluster
-            if (pulsar.getLoadManager() != null) {
+            if (pulsar.getLoadManager() != null && pulsar.getLoadManager().get() != null) {
                 try {
                     pulsar.getLoadManager().get().disableBroker();
                 } catch (PulsarServerException.NotFoundException ne) {