MINOR Fix CI workflow for push event (#17239)

On trunk, our CI runs in response to "push" events. The change in #17227 causes the workflow template to be invalid, which prevents the build from starting. This patch fixes that by defaulting to `false`

Reviewers: Justine Olshan <[email protected]>

.github/workflows/ci.yml

@@ -35,6 +35,6 @@ jobs:
     with:
       gradle-cache-read-only: ${{ github.ref != 'refs/heads/trunk' }}
       gradle-cache-write-only: ${{ github.ref == 'refs/heads/trunk' }}
-      is-public-fork: ${{ github.event.pull_request.head.repo.fork }}
+      is-public-fork: ${{ github.event.pull_request.head.repo.fork || false }}
     secrets:
       inherit