Will get null message when locale is not default even if there is default messages.properties

For instance, if the locale is en_US and only messages.properties is offered, null will be rendered like this:

```bash
#classpath:messages.properties
some_code: Something
```

Error result:
```javascript
{
  "errors": [
    {
      "code": "some_code",
      "message": "null"
    }
  ]
}
```

The result should grab the default message when its locale is not found.