 The value is converted to the json string when set `UNDEFINED_TO_STRING` to `true`.    For example, if not set `UNDEFINED_TO_STRING` , we may see a field's value like:   "Number": 10,  10 is an integer here  but if set `UNDEFINED_TO_STRING` to `true`, then the value should be:  "Number": "10",  "10" is a string here