Prevent unhandled exception when running via schedule (#69)

* Prevent unhandled exception when running via schedule

pull-request.py

@@ -394,7 +394,7 @@ def main():
     if not from_branch:
         print("PULL_REQUEST_FROM_BRANCH is not set, checking branch in payload.")
         with open(check_events_json(), "r") as fd:
-            from_branch = json.loads(fd.read()).get("ref")
+            from_branch = json.loads(fd.read()).get("ref", "")
         from_branch = from_branch.replace("refs/heads/", "").strip("/")
     else:
         print("PULL_REQUEST_FROM_BRANCH is set.")