`VaultCreate` txn improperly serialized with `MPToken` Asset input

[ckeshava]

The xrpl-py client library does not correctly serialize VaultCreate transaction when an MPToken is used as the Asset of the vault. The improper serialization of the fields result in a rippled error that looks in this fashion:

{'api_version': 2, 'error': 'invalidTransaction', 'error_exception': 'invalid SerialIter getBitString', 'id': 'RequestMethod.SUBMIT_381981', 'request': {'api_version': 2, 'command': 'submit', 'fail_hard': False, 'id': 'RequestMethod.SUBMIT_381981', 'tx_blob': '120041210000F7E02400000450201B000004646840000000004C4B407321ED13CC1FDE94B78FBC6840457512A7441362D311F58E4BADFF6BDF7BECEB16157E744081EFA5E2A27112AD7CB9B10EC4BAA45CCE465B1F8D3877ADF9989AC43812089155F9BAB7795E4AA8B05AA3F330468546822931DC7BDED3F5707C0A9241C1B203811499F70D70085D423EBB0F75CCA90FF117993B296903180000044F99F70D70085D423EBB0F75CCA90FF117993B2969'}, 'status': 'error', 'type': 'response'}

The input transaction (in JSON format) looks like this:

{'Account': 'rEsaLimTG68Dks7cA3UWUswNiYrDCCAcrD', 'TransactionType': 'VaultCreate', 'Fee': '5000000', 'Sequence': 1104, 'LastLedgerSequence': 1124, 'SigningPubKey': 'ED13CC1FDE94B78FBC6840457512A7441362D311F58E4BADFF6BDF7BECEB16157E', 'TxnSignature': '81EFA5E2A27112AD7CB9B10EC4BAA45CCE465B1F8D3877ADF9989AC43812089155F9BAB7795E4AA8B05AA3F330468546822931DC7BDED3F5707C0A9241C1B203', 'NetworkID': 63456, 'Asset': {'mpt_issuance_id': '0000044F99F70D70085D423EBB0F75CCA90FF117993B2969'}}

This error originates from this location inside rippled.

The client library does not serialize the transaction in a manner that is compatible with the XRPL Binary Format protocol. rippled serialized STIssue (with MPToken) in the following way:

 - 160 bits MPT issuer account
 - 160 bits black hole account
 - 32 bits sequence

xrpl-py serializes the STIssue type in a different way:

 - 160 bits MPT issuer account
 - 32 bits sequence
(a total of 192 bits)

The lack of black-hole account serialization in xrpl-py causes the mismatch in the XRPL binary format expectations.