store offline messages with empty body for pubsub

SVN 13344

This fix seems to work:

if (message.getBody() == null || message.getBody().length() == 0) {
// ignore empty bodied message (typically chat-state notifications).
// but not pubsub messages
if(message.getChildElement("event", "http://jabber.org/protocol/pubsub#event") == null){
return;
}
}