ci: run wpt classic only on PR

Author: krichprollsch

.github/workflows/wpt.yml

@@ -41,8 +41,8 @@ jobs:
   wpt:
     name: web platform tests
 
-    # Don't run the CI with draft PR.
-    if: github.event.pull_request.draft == false
+    # Only for PR without draft.
+    if: github.event_name == 'pull_request' && github.event.pull_request.draft == false
 
     runs-on: ubuntu-latest