fix: update GitHub Actions workflow to fallback to 'main' branch if b…

…ase ref is not set

.github/workflows/test.yml

@@ -42,7 +42,7 @@ jobs:
         ref: ${{ github.event.pull_request.head.ref }}
         repository: ${{ github.event.pull_request.head.repo.full_name }}
     - name: Fetch target branch
-      run: git fetch origin ${{ github.event.pull_request.base.ref}}:${{ github.event.pull_request.base.ref}}
+      run: git fetch origin ${{ github.event.pull_request.base.ref || 'main'}}:${{ github.event.pull_request.base.ref || 'main'}}
     - name: Check changes
       id: check
       run: |