docs: def serialize_on_parent(
              self,
              parent,  # type: ET.Element
              value,  # type: Any
              state  # type: _ProcessorState
      ):
          # type: (...) -> None
          """Serialize the value and add it to the parent element."""
          # Note that falsey values are not treated as missing, but they may be omitted.
          if value is None and self.required:
              state.raise_error(MissingValue, self._missing_value_message(parent))
  
          if not value and self.omit_empty:
              return  # Do Nothing
  
          element = _element_get_or_add_from_parent(parent, self.element_path)
          self._serialize(element, value, state)