add padding to the frame if required.

        note that padding is only added if pay is None/empty. this allows us to add  # noqa: E501
        any payload after the MACControl* PDU if needed (piggybacking).

def post_build(self, pkt, pay):
        """
        add padding to the frame if required.

        note that padding is only added if pay is None/empty. this allows us to add  # noqa: E501
        any payload after the MACControl* PDU if needed (piggybacking).
        """

        if not pay:
            under_layers_size = self._get_underlayers_size()

            frame_size = (len(pkt) + under_layers_size)

            if frame_size < 64:
                return pkt + b'\x00' * (64 - frame_size)

        return pkt + pay