Fix test time history report if no ancestor report (#52054)
Summary:
This fixes an issue (currently blocking https://github.com/pytorch/pytorch/issues/51905) where the test time regression reporting step will fail if none of the most recent `master` ancestors have any reports in S3 (e.g. if a new job is added).
Pull Request resolved: https://github.com/pytorch/pytorch/pull/52054
Test Plan:
```
python test/test_testing.py
```
Reviewed By: walterddr
Differential Revision: D26369507
Pulled By: samestep
fbshipit-source-id: 4c4e1e290cb943ce8fcdadacbf51d66b31c3262a
