Fix BadRequest raised with empty body and application/json content type

A ReqParser behavior change was introduced in 49502376 as a  side-effect.

Before 49502376, ``ReqParser`` was able to handle request with empty body and ``content_type='application/json'`` if none of the arguments where expected in the JSON.
After 49502376, ``ReqParser`` raise a BadRequest try to read ``request.json``, even if none of the argument has ``location='json'``, even with ``strict=False``

This pull-request restore this behavior when ``strict=False``