TIM-548

[TIM-548](https://edx-wiki.atlassian.net/browse/TIM-548)
Bugfix: 500 error when clicking "peer assess" with no submission.

The `workflow is None` line was -- hopefully -- never being executed, since it returned a `Response` object instead of a `(path, context)` tuple, which would have caused a 500 error.

@stephensanchez