Fix a exception being thrown from Diviner
Summary: If no Diviner groups are defined that this line currently throws a fatal exception... make this a little more safe.
Test Plan: Applied to our install, no more exceptions.
Reviewers: epriestley, Blessed Reviewers
Reviewed By: epriestley, Blessed Reviewers
Subscribers: Korvin, epriestley
Differential Revision: https://secure.phabricator.com/D10999