Set a property so that unit tests run on PHP7

Summary: Without this change PHP throws because idx() is passed null as the property is not intialzied

Test Plan: arc unit --everything

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: epriestley

Differential Revision: https://secure.phabricator.com/D14345