parse_object(): simplify blob conditional

parse_object(): simplify blob conditional

Commit 8db2dad7a0 (parse_object(): check on-disk type of suspected blob,
2022-11-17) simplified the conditional for checking if we might have a
blob. But we can simplify it further. In:

  !obj || (obj && obj->type == OBJ_BLOB)

the short-circuit "OR" means "obj" will always be true on the right-hand
side. The compiler almost certainly optimized that out anyway, but
dropping it makes the conditional easier to understand for humans.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>