Fix error when trying to unregister an account from a client

java.lang.IllegalArgumentException: Adding text to an XML document must not be null
at org.dom4j.DocumentFactory.createText(DocumentFactory.java:174)
at org.dom4j.tree.AbstractElement.addText(AbstractElement.java:899)
at org.dom4j.tree.AbstractElement.setText(AbstractElement.java:1120)
at org.jivesoftware.wildfire.handler.IQRegisterHandler.handleIQ(IQRegisterHandler.java:173)
at org.jivesoftware.wildfire.handler.IQHandler.process(IQHandler.java:48)
at org.jivesoftware.wildfire.IQRouter.handle(IQRouter.java:268)
at org.jivesoftware.wildfire.IQRouter.route(IQRouter.java:96)