Empty response body (3-rd argument) must be passed to callback as an empty string

Docs sates: "The callback argument gets 3 arguments. ... The third is the response body String or Buffer."
If response has an empty body (Content-length is 0) third argument is `undefined`, not `String` nor `Buffer`.

The "edge case" is JSON response. Empty body can't be parsed to something convinient, so, this case still results in `undefined` (may be it must be handled different way?).