[CustomDescriptors] Ensure a non-nullable type for br_on_cast_desc_eq on null (#8975)

If the desc is null, the cast is not even executed, making it
unreachable. We typed it as nullable if the ref was (in the code path `type =
ref->type;`), but emiting an uninhabitable type is more precise, and also avoids a
fuzz bug (see new test) where refinalize actually un-refined a type: it
turned it from non-nullable to nullable.