[SingleStream] null response for play_later when card_data is empty

[rerenoob]

def play_later(url, card_data, response_builder):
    """Play the stream later.

    https://developer.amazon.com/docs/custom-skills/audioplayer-interface-reference.html#play
    REPLACE_ENQUEUED: Replace all streams in the queue. This does not impact the currently playing stream.
    """
    # type: (str, Dict, ResponseFactory) -> Response
    if card_data:
        # Using URL as token as they are all unique
        response_builder.add_directive(
            PlayDirective(
                play_behavior=PlayBehavior.REPLACE_ENQUEUED,
                audio_item=AudioItem(
                    stream=Stream(
                        token=url,
                        url=url,
                        offset_in_milliseconds=0,
                        expected_previous_token=None),
                    metadata=add_screen_background(card_data)))
        ).set_should_end_session(True)

        return response_builder.response

The above function will return an invalid response if card_data is empty. The function should still return a valid response.