[GHF] Fix pushDate handling (#94364)
Merge commits does not have a merge date, which is also clear from [GraphQL schema](https://docs.github.com/en/graphql/reference/objects#commit).
Modify return signature of `GitHubPR.last_pushed_at`, print warning when one can not be queried and add regression test.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/94364
Approved by: https://github.com/huydhn
