[bug 971014] `Manager.get_empty_query_set() was removed. Use `none()` instead

This fixes: https://errormill.mozilla.org/support/sumo-stage/group/166295/

I added a test that triggers the issue, then fixed it.

Also see: https://github.com/django/django/commit/fb606e10ac07af6014cae838c21bf30cf50e8b40

Small r?