Don't throw error when user doesn't have last_name set

#### Any background context you want to provide?
There are a few random sprintly users with blank `last_name` fields. Additionally, with the new github signup, it's possible for more users with only the `first_name` field set.

#### What's this PR do?
Use the `format_name` helper where appropriate. Modify `format_name` to handle users with blank `last_name`. Add test.

#### Definition of Done:
- [x] Is there appropriate test coverage? (e.g. ChefSpec, Mocha/Chai, Python, etc.)