ci: fix github head ref

Author: farioas

.github/workflows/tests.yml

@@ -154,7 +154,7 @@ jobs:
         uses: actions/github-script@v7
         env:
           BASE_REF: "${{ github.event.pull_request.base.sha || github.event.before }}"
-          HEAD_REF: "${{ github.event.pull_request.head.sha || github.event.after }}"
+          HEAD_REF: "${{ github.event.pull_request.head.sha || github.event.after || github.sha }}"
           EVENT_NAME: "${{ github.event_name }}"
         with:
           script: |