Fixes NPE when sending null as the node while trying to get the NodeInformationProvider for a node.

git-svn-id: http://svn.igniterealtime.org/svn/repos/smack/trunk@2364 b35dd754-fafc-0310-a699-88a17e54d16e

source/org/jivesoftware/smackx/ServiceDiscoveryManager.java

@@ -248,6 +248,9 @@ public class ServiceDiscoveryManager {
      * to a given node.
      */
     private NodeInformationProvider getNodeInformationProvider(String node) {
+        if (node == null) {
+            return null;
+        }
         return (NodeInformationProvider) nodeInformationProviders.get(node);
     }