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