Fix NPE in DiscoverInfo.Identities XmlStringBuilder

The 'name' attribute of an service discovery identity is
optional. Thanks to Jens Offenbach for reporting.

smack-extensions/src/main/java/org/jivesoftware/smackx/disco/packet/DiscoverInfo.java

@@ -327,7 +327,7 @@ public class DiscoverInfo extends IQ implements Cloneable {
             xml.halfOpenElement("identity");
             xml.xmllangAttribute(lang);
             xml.attribute("category", category);
-            xml.attribute("name", name);
+            xml.optAttribute("name", name);
             xml.optAttribute("type", type);
             xml.closeEmptyElement();
             return xml;