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