Handle case when no new data is available

.github/workflows/generate-map-data.yaml

@@ -36,7 +36,7 @@ jobs:
           git config user.email "action@github.com"
           git config user.name "GitHub Actions"
           git add inst/extdata/map_data.json
-          git commit -m "Fetch map data"
+          git diff-index --quiet HEAD || git commit -m "Fetch map data"
           git pull --rebase origin main
           git push
           echo "pushed to github"