When there are no events in a timleine, render a single spacer

Summary: Ref T2222. I accidentally changed the beahvior of this in D8229.

Test Plan: Revisions with no comments no longer with too little space between the main object box and the "Revision Update History".

Reviewers: chad, btrahan

Reviewed By: chad

CC: aran

Maniphest Tasks: T2222

Differential Revision: https://secure.phabricator.com/D8239