workflows/backport: avoid broken korthout/backport-action output

Somehow, the was_successful output didn't work correctly. It's hard to
imagine that the created_pull_numbers output fails the same way,
because... when the backport action fails there **are no pull request
numbers**.

.github/workflows/backport.yml

@@ -46,7 +46,7 @@ jobs:
               * Even as a non-commiter, if you find that it is not acceptable, leave a comment.
 
       - name: "Add 'has: port to stable' label"
-        if: steps.backport.outputs.was_successful
+        if: steps.backport.outputs.created_pull_numbers != ''
         env:
           GH_TOKEN: ${{ steps.app-token.outputs.token }}
           REPOSITORY: ${{ github.repository }}