Defensive programming for unit-test result publising.

Make it slightly more robust, since there have been problems with this in the past aparently (see github.com/ations/runner/1722). Manual rewrite of earlier pull-request Cura/pull/13497

Co-authored-by: 0r31 <orel@dagoma.fr>

.github/workflows/unit-test-post.yml

@@ -79,4 +79,4 @@ jobs:
                     files: "tests/**/*.xml"
 
             -   name: Conclusion
-                run: echo "Conclusion is ${{ fromJSON( steps.test-results.outputs.json ).conclusion }}"
+                run: echo "Conclusion is ${{ steps.test-results.outputs.json && fromJSON( steps.test-results.outputs.json ).conclusion }}"