Fix regression with raw ResponseEntity type

This fix addresses a 4.1.1 regression where a raw ResponseEntity return
value (used to return potentially a different kind of body) caused an
exception.

The regression came from the fact we now try to render a null body in
order to give ResponseBodyAdvice a chance to substitute a different
value. That in turn means we have to try to determine the body type
from the method signature.

This change improves the logic for extracting the generic parameter
type to accommodate a raw ResponseEntity class. Also we avoid raising
HttpMediaTypeNotAcceptableException if the value to be rendered is
null.

Issue: SPR-12287