Don't return a 415 when the body is empty

This is a solution for gratipay/gratipay.com#2991 and gratipay/gratipay.com#3004. It seems that when jQuery is asked to make an empty POST request, it doesn't set a `Content-Type`, so Aspen complains about not being able to parse the body, but since there is nothing to parse anyway I think returning an empty dict is a better idea.