Return the secret for the transfer, None on EMPTY_SECRET.

def secret_from_transfer_task(
        transfer_task: Optional[TransferTask],
        secrethash: SecretHash,
) -> Optional[Secret]:
    """Return the secret for the transfer, None on EMPTY_SECRET."""
    assert isinstance(transfer_task, InitiatorTask)

    transfer_state = transfer_task.manager_state.initiator_transfers[secrethash]

    if transfer_state is None:
        return None

    return transfer_state.transfer_description.secret