Fix the case when workflow is empty (#1438)
Summary:
The workflow shouldn't be broken if the workflows to run is empty.
Pull Request resolved: https://github.com/pytorch/benchmark/pull/1438
Test Plan: workflow: https://github.com/pytorch/benchmark/actions/runs/4284353521
Reviewed By: davidberard98
Differential Revision: D43625674
Pulled By: xuzhao9
fbshipit-source-id: 386607107786d23029a13913306fc41df19371f7
