fix(ci): validate the porting record after force pushes

Fall back to the default branch when the event baseline is absent from the checkout. Preserve available comparison commits and always run the validator.

Validation: the actual workflow shell passes valid, empty, new-branch and unavailable force-push baseline cases.

.github/workflows/porting-record.yml

@@ -28,7 +28,8 @@ jobs:
           BASE: ${{ github.event.pull_request.base.sha || github.event.before }}
           DEFAULT_BRANCH: ${{ github.event.repository.default_branch }}
         run: |
-          if [[ -z "$BASE" || "$BASE" =~ ^0+$ ]]; then
+          if [[ -z "$BASE" || "$BASE" =~ ^0+$ ]] ||
+             ! git -C device cat-file -e "${BASE}^{commit}" 2>/dev/null; then
             BASE="refs/remotes/origin/$DEFAULT_BRANCH"
           fi
           python3 tools/scripts/porting-record.py check --device device --base "$BASE"