Add missing username nullable message for RegisterCommand.

When creating an account from the `register/index` view, if a blank form is submitted, all fields display correct error messages except for username (shown below):

![screenshot from 2014-11-28 08 29 48](https://cloud.githubusercontent.com/assets/3237612/5228533/5d69d5d2-76d9-11e4-88e0-0de1100bdc48.png)

This commit resolves this issue.