Throw a ConstraintViolationException for empty resources

Throw a `ConstraintViolationException` (although with no `ConstraintViolation<>` at all) if an empty resource is being validated. At least a response with status 422 (Unprocessable Entity) is being generated by `ConstraintViolationExceptionMapper`. This kind of fixes #431.

Unfortunately it seems to be very hard to create proper `ConstraintViolation<T>` instances without rewriting half of the validation framework.