Dont fail if submodule already at given ref

Author: evertlammerts

.github/workflows/on_external_dispatch.yml

@@ -50,6 +50,10 @@ jobs:
           git config user.name "github-actions[bot]"
           git config user.email "41898282+github-actions[bot]@users.noreply.github.com"
           git add external/duckdb
+          if git diff --cached --quiet; then
+            echo "No changes to commit: submodule ref is unchanged."
+            exit 0
+          fi
           git commit -m "Update submodule ref"
           git push