fix(ci): null-safe files iteration in the paginated compare (#66867)

A PR more than 100 commits ahead of its merge-base paginates the compare
endpoint; pages after the first carry files: null, so the bare .files[]
made jq die with 'cannot iterate over: null' on every retry and forced
the classifier's fail-open path (all lanes on, ci_review gate demanding
a label with zero CI-sensitive files in the diff). .files[]? keeps the
page-one file list and ignores the null tail.

.github/actions/detect-changes/action.yml

@@ -72,12 +72,19 @@ runs:
           # Retried: a rate-limit blip or eventual-consistency 404 on a
           # freshly-pushed HEAD would otherwise silently fall open (all lanes
           # run — safe, but wasteful and it masks the API failure).
+          #
+          # `.files[]?` (null-safe): with --paginate, a PR more than 100
+          # commits ahead of its merge-base paginates the compare, and pages
+          # after the first carry `files: null` — bare `.files[]` makes jq
+          # die with "cannot iterate over: null", which fails every retry
+          # and forces the fail-open path (seen on stacked PRs). The full
+          # file list (up to the API's 300-file cap) is on page one.
           CHANGED=""
           for i in 1 2 3; do
             if CHANGED="$(gh api \
               --paginate \
               "repos/${REPO}/compare/${BASE_SHA}...${HEAD_SHA}" \
-              --jq '.files[].filename')"; then
+              --jq '.files[]?.filename')"; then
               break
             fi
             if [ "$i" = 3 ]; then