fix accept header handling when client sends no accept header

we discovered this in a rest api where a client was sending requests without accept headers. this is not good practice but allowed by the http spec. if the backend only has one type of answer, it usually works out.

we accidentally converted this into an existing Accept header with empty value, which by the HTTP spec means the client is not accepting any format. leading a spec-adherent backend with no other option than raising a 4xx error.