message: Return null pointer as the VmCall output if Burrow returns nil
diff: @@ -107,6 +107,9 @@ func VmCall(input, caller, callee, nonce *C.const_char, commandExecutor, queryEx
  if err != nil {
  return makeError(err.Error())
  }
  + if output == nil {
  + return nil, nil
  + }
  return C.CString(hex.EncodeToString(output)), nil
  }
  