Without a Reviewers line, cpr still tries to set reviewers

We should skip the set-reviewers step when no reviewers are supplied.

From the output of cpr without a reviewers line:
```
Sending pull request...
https://github.com/dydxprotocol/protocol/pull/435 (copied to clipboard)
Setting reviewers...
Validation Failed
Could not add requested reviewers to pull request.
```