[3.12] gh-121660: Fix `ga_getitem` by explicitly checking for `NULL` result (GH-121661) (#121762)

gh-121660: Fix `ga_getitem` by explicitly checking for `NULL` result (GH-121661)
(cherry picked from commit bb802db8cfa35a88582be32fae05fe1cf8f237b1)

Co-authored-by: sobolevn <mail@sobolevn.me>