update workflow: commit only on flake changes

.forgejo/workflows/update.yaml

@@ -16,10 +16,8 @@ jobs:
           git config --global user.name "actions_user"
       - name: update flake.lock
         run: nix --extra-experimental-features "nix-command flakes" flake update
-      - name: diff flake.lock
-        run: git diff --exit-code flake.lock && exit 0
       - name: commit and push update
         run: |
-          git add .
+          git diff --exit-code flake.lock && exit 0 || echo ""
           git commit -m "update flake.lock"
           git push "https://actions_user:${{ secrets.WRITE_TOKEN }}@git.toufy.me/toufy/adonis" "main"