don't allow creating empty PRs (#23072)

Part of #998

Author: Peter Bengtsson

.github/workflows/repo-sync.yml

@@ -128,6 +128,9 @@ jobs:
           pr_body: "This is an automated pull request to sync changes between the public and private repos.\n\n:robot: This pull request should be merged (not squashed) to preserve continuity across repos, so please let a bot do the merging!"
           pr_label: automated-reposync-pr
           github_token: ${{ secrets.OCTOMERGER_PAT_WITH_REPO_AND_WORKFLOW_SCOPE }}
+          # This will exit 0 if there's no difference between `repo-sync`
+          # and `main`. And if so, no PR will be created.
+          pr_allow_empty: false
 
       - name: Find pull request
         uses: juliangruber/find-pull-request-action@db875662766249c049b2dcd85293892d61cb0b51