wallet: Replace boost::none with nullopt
wallet: Replace boost::none with nullopt

Former-commit-id: b27350d68e6aabf10a7b20ecd892a21b8aeed12b
syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin,syscoin/syscoin