Add explicit null check for better clarity and robustness

Co-authored-by: Arash-Sabet <26050123+Arash-Sabet@users.noreply.github.com>

Author: Copilot

.github/workflows/auto-assign.yml

@@ -8,7 +8,7 @@ jobs:
   run:
     runs-on: ubuntu-latest
     # Skip this workflow for pull requests from forks as GITHUB_TOKEN lacks write permissions
-    if: github.event_name == 'issues' || github.event.pull_request.head.repo.full_name == github.repository
+    if: github.event_name == 'issues' || (github.event.pull_request != null && github.event.pull_request.head.repo.full_name == github.repository)
     permissions:
       issues: write
       pull-requests: write