[mergebot][BE] If we don't have any required checks, don't run required checks (#121921)
This PR addresses the issue identified in #121920. The existing problem is that all tests are deemed mandatory if none are selected as required. This behavior is particularly noticeable during a force merge operation.
In the context of a force merge, it may not be necessary to execute any tests which are not required (imo). However, this proposed change could be seen as controversial, hence it has been separated from the main update for further discussion and review.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/121921
Approved by: https://github.com/huydhn
ghstack dependencies: #121920
