code: protected String getOptionalAttributeValue(StartElement start,
                                                 QName attributeName) {
          Attribute mapNameAttribute = start.getAttributeByName(attributeName);
          if (mapNameAttribute == null) {
              return null;
          } else {
              return mapNameAttribute.getValue();
          }
      }
docs: Get the value of a given attribute from this start tag, or null if the
  attribute is not there.