Find a transaction on the distributed ledger by its sequence number.

        :param txn: transaction number
        :return: json sequence number of transaction, null for no match

async def get_txn(self, txn: int) -> str:
        """
        Find a transaction on the distributed ledger by its sequence number.

        :param txn: transaction number
        :return: json sequence number of transaction, null for no match
        """

        LOGGER.debug('_BaseAgent.get_txn >>> txn: %s', txn)

        rv_json = json.dumps({})
        req_json = await ledger.build_get_txn_request(self.did, None, txn)
        resp = json.loads(await self._submit(req_json))

        rv_json = json.dumps((resp['result'].get('data', {}) or {}).get('txn', {}))  # "data": null for no such txn
        LOGGER.debug('_BaseAgent.get_txn <<< %s', rv_json)
        return rv_json