spec. Section 4.2.4 says:
When reading zero-length request entities all pre-packaged MessageBodyReader implementations, except the JAXB-related one and those for the (boxed) primitive types above, MUST create a corresponding Java object that represents zero-length data; they MUST NOT return null. The pre-packaged JAXB and the pre- packaged primitive type MessageBodyReader's MUST throw a BadRequestException (400 status) for zero-length request entities.
returns "NULL" for request:
>> POST http://localhost:8080/web/resource/jaxb
>> Accept:/, Content-Type:application/xml,