code: public static String serializeMap(Map<String, String> map) {
          if (map == null) {
              map = new HashMap<String, String>();
          }
          XStream xstream = new XStream(new DomDriver());
          return xstream.toXML(map);
      }
docs: Serializes a Map to XML. If the map is either empty or null
  the XML will indicate an empty map.
  
  @param map
  @return