test_dep_update: exit gracefully if no pending PRs

.github/workflows/test_dep_update.sh

@@ -7,6 +7,12 @@ git branch depupdate
 git checkout depupdate
 git reset --hard origin/master
 json="$(gh pr list --search "author:app/github-actions" --json headRefName,number)"
+# If there's no updates then the CI action will fail without this, which is more common if we reduce the schedule from 24 hours.
+if [[ $json == "[]" ]]; then
+    echo "No Dependency update PRs to merge"
+    exit
+fi
+
 echo "$json" | jq '.[] | .headRefName | @text' | xargs -L1 -- git pull origin
 if nix build .#checks.x86_64-linux.init-example-el; then
     echo "$json" | jq ".[] | .number | @text" | xargs -L1 -- gh pr merge --squash --delete-branch