Applying github-review-approve with no comments fails

I can't quite follow the logic of github-review-submit-review but I can't see it calling either github-review-post-review-replies or github-review-post-review when I haven't added any additional comments and I just want to approve the PR. Some modest instrumenting of the two legs and I see:

```
Submitting review, this may take a while ...
ghr-prr: t nil
ghr-rpr: nil nil
```
