Fix docathon-label-sync.py to not fail on PRs without description (#2379)

See https://github.com/pytorch/tutorials/actions/runs/5140794478/jobs/9252588225?pr=2377 as an example

Author: kit1980

Diff for: .github/scripts/docathon-label-sync.py

@@ -14,6 +14,9 @@ def main():
     repo = g.get_repo(f'{repo_owner}/{repo_name}')
     pull_request = repo.get_pull(pull_request_number)
     pull_request_body = pull_request.body
+    # PR without description
+    if pull_request_body is None:
+        return
 
     # get issue number from the PR body
     if not re.search(r'#\d{1,5}', pull_request_body):