Serialize the value and add it to the parent element.

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)