fix rendering of uncategorized account level groups

fixes CNVS-14243

test plan: create an Uncategorized account-level group, easiest way to
do this is a SIS import like so:

group_id,name,status
fromsis,from SIS,available

Then visit /accounts/self/groups. The group should appear in the
"Uncategorized" tab. Visit the group homepage /groups/:id by clicking
the gear -> "Visit Group Homepage". The group homepage should render
without error, and you should be able to interact with the group like
any other. Test this as a student enrolled in the group, as well.

Change-Id: Id4f08b704226653acc35df517fe58ce4f8f9e6eb
Reviewed-on: https://gerrit.instructure.com/40789
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Brian Palmer <brianp@instructure.com>