Fix error when shared group has no display name

org.jivesoftware.wildfire.event.GroupEventDispatcher.dispatchEvent(GroupEventDispatcher.java:142)
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.xmpp.packet.Roster.addItem(Roster.java:189)
at org.jivesoftware.wildfire.roster.Roster.broadcast(Roster.java:688)
at org.jivesoftware.wildfire.roster.Roster.addSharedUser(Roster.java:815)
at org.jivesoftware.wildfire.roster.RosterManager.groupUserAdded(RosterManager.java:595)
at org.jivesoftware.wildfire.roster.RosterManager.groupUserAdded(RosterManager.java:561)
at org.jivesoftware.wildfire.roster.RosterManager.groupModified(RosterManager.java:298)
at org.jivesoftware.wildfire.event.GroupEventDispatcher.dispatchEvent(GroupEventDispatcher.java:134)
at org.jivesoftware.wildfire.group.Group$PropertiesMap.put(Group.java:463)
at org.jivesoftware.wildfire.admin.group_002dedit_jsp._jspService(group_002dedit_jsp.java:164)
at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:94)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:689)
at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:428)