Fix NPE in Node.java when changing node configuration.

This NPE caused two issues:
- subscribers are not correctly notified
- change is not persisted to the DB

https://community.igniterealtime.org/thread/57584

src/java/org/jivesoftware/openfire/pubsub/Node.java

@@ -875,7 +875,7 @@ public abstract class Node {
             formField.setLabel(LocaleUtils.getLocalizedString("pubsub.form.conf.collection"));
         }
 
-        if (!parent.isRootCollectionNode()) {
+        if (parent != null && !parent.isRootCollectionNode()) {
         	formField.addValue(parent.getNodeID());
         }