docs: def _set_nil(self, element, value_parser):
          """
          Method to set an attribute of the element.
          If the value of the field is None then set the nil='true' attribute in the element
  
          :param element: the element which needs to be modified
          :type element: xml.etree.ElementTree.Element
          :param value_parser: the lambda function which changes will be done to the self.value
          :type value_parser: def
          :return: the element with or without the specific attribute
          :rtype: xml.etree.ElementTree.Element
          """
          if self.value:
              element.text = value_parser(self.value)
          else:
              element.attrib['nil'] = 'true'
          return element