fix: invalid hexlify value

[vivianjeng]

The gas in decimal would not be valid in some case

/Users/zhengyawen/Documents/GitHub/Unirep/node_modules/solc/soljson.js:133
    process["on"]("unhandledRejection", function (reason) { throw reason; });
                                                            ^

Error: invalid hexlify value (argument="value", value="10000000000000000", code=INVALID_ARGUMENT, version=bytes/5.7.0)
    at Logger.makeError (/Users/zhengyawen/Documents/GitHub/Unirep/node_modules/@ethersproject/logger/lib/index.js:238:21)
    at Logger.throwError (/Users/zhengyawen/Documents/GitHub/Unirep/node_modules/@ethersproject/logger/lib/index.js:247:20)
    at Logger.throwArgumentError (/Users/zhengyawen/Documents/GitHub/Unirep/node_modules/@ethersproject/logger/lib/index.js:250:21)
    at hexlify (/Users/zhengyawen/Documents/GitHub/Unirep/node_modules/@ethersproject/bytes/lib/index.js:200:19)
    at /Users/zhengyawen/Documents/GitHub/Unirep/node_modules/@ethersproject/transactions/lib/index.js:181:59
    at Array.forEach (<anonymous>)
    at _serialize (/Users/zhengyawen/Documents/GitHub/Unirep/node_modules/@ethersproject/transactions/lib/index.js:175:23)
    at serialize (/Users/zhengyawen/Documents/GitHub/Unirep/node_modules/@ethersproject/transactions/lib/index.js:246:16)
    at /Users/zhengyawen/Documents/GitHub/Unirep/node_modules/@ethersproject/wallet/lib/index.js:163:116
    at processTicksAndRejections (node:internal/process/task_queues:96:5) {
  reason: 'invalid hexlify value',
  code: 'INVALID_ARGUMENT',
  argument: 'value',
  value: '10000000000000000'
}