Fix Phriction toc rendering when toc is null

Summary: Removes the exception, maybe there is a better way, but landing this for now. Fixes T9829

Test Plan: Test pages with and without a table of contents

Reviewers: epriestley, avivey

Subscribers: epriestley

Maniphest Tasks: T9829

Differential Revision: https://secure.phabricator.com/D14546