Check if `held[asset]` equal None.
Check if `held[asset]` equal None.

stefcrypto/counterparty-lib,CounterpartyXCP/counterparty-lib,coinwarp/dogeparty-lib,monaparty/counterparty-lib,F483/counterparty-lib,F483/counterparty-lib,tokenly/counterparty-lib,CounterpartyXCP/counterparty-lib,jarymoth/counterpartyd,monaparty/counterparty-lib