Fixed parsing problem when NULL string was cached.

git-svn-id: http://svn.igniterealtime.org/svn/repos/wildfire/trunk@6361 b35dd754-fafc-0310-a699-88a17e54d16e

src/java/org/jivesoftware/wildfire/spi/PresenceManagerImpl.java

@@ -321,7 +321,7 @@ public class PresenceManagerImpl extends BasicModule implements PresenceManager
                         loadOfflinePresence(probee.getNode());
                     }
                     presenceXML = offlinePresenceCache.get(probee.getNode());
-                    if (presenceXML != null) {
+                    if (presenceXML != null && !NULL_STRING.equals(presenceXML)) {
                         try {
                             // Parse the element
                             Document element = DocumentHelper.parseText(presenceXML);