bug when not using --source-org and --source-branch on PR command

@Padam87 you did not check the command without arguments if it was still working

it is not working, can you please launch a PR fixing this? Thanks

```
  [Github\Exception\ValidationFailedException]
  Validation Failed: Field "head_sha" is missing, for resource "PullRequest", Field "base_sh
  a" is missing, for resource "PullRequest", No commits between gushphp:master and gushphp:8
  9-it-is-either-branch-push-or-b-fork-or-other-that-is-tracking-the-master-branch

```