OF-1467, Check nodeRoutes.containsKeys(key) to avoid NPE (#999)

* OF-1467, Check nodeRoutes.containsKeys(key) to avoid Null Pointer Exception in ClusterListener, nodeCleanup, when there are no sessions on the node to cleanup

* Good point. Tweak to fix as per suggestion.

src/plugins/hazelcast/src/java/org/jivesoftware/openfire/plugin/util/cache/ClusterListener.java

@@ -280,13 +280,14 @@ public class ClusterListener implements MembershipListener, LifecycleListener {
 
         // Remove outgoing server sessions hosted in node that left the cluster
         Set<DomainPair> remoteServers = nodeRoutes.get(key);
-        if (!remoteServers.isEmpty()) {
+        if (remoteServers!=null) {
             for (DomainPair domainPair : remoteServers) {
                 routingTable.removeServerRoute(domainPair);
             }
         }
         nodeRoutes.remove(key);
 
+
         Set<String> components = lookupJIDList(key, componentsCache.getName());
         if (!components.isEmpty()) {
             for (String address : new ArrayList<String>(components)) {