Hide repository from atom view if it hasn't been set

Summary: Fixes T8626. I guess that $this->getViewer()->renderHandle(null) renders as "Unknown Object", which is somewhat odd to me.

Test Plan: Viewed an atom from a book without a repository linked to it.

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley, #blessed_reviewers

Subscribers: epriestley, Korvin

Maniphest Tasks: T8626

Differential Revision: https://secure.phabricator.com/D13370