Improve support for running on push event

[andrewmcnamara]

Improves support for push events. Original issue that has been closed unresolved

On initial PR creation the base_ref is null .

{
  "after": "41da3d31e13c738899d9b167156d4[22](https://github.com/andrewmcnamara/jacoco-playground/actions/runs/6347324298/job/17242166029#step:7:23)875194b15",
  "base_ref": null,
  "before": "0000000000000000000000000000000000000000",
 
} 
  

This causes client.rest.repos.compareCommits in getChangedFiles to fail with HttpError: Not Found as the base ref is not found.

This PR surrounds this call with a try/catch and additionally makes an assumption that what you really want is to compare against your default branch. Which is correct in most cases.