return 204 on empty query set for ListModelMixin on comm (bug 912239)

By default, on an empty queryset, ListModelMixin returns 500 (EmptyResultsSet).

Given an allow_empty flag, ListModelMixin instead returns 404.

For Commbadge purposes, we prefer a plain empty Response.

![screen shot 2013-09-04 at 4 41 07 pm](https://f.cloud.github.com/assets/674727/1084925/ec4fb094-15bb-11e3-9d7c-6824aacdfac1.png)