Don't show a blank line if there is no user data

Summary:
SKIP lines, for instance, often have no UserData; there is no
reason to display a content-less blank line.

Test Plan: arc unit

Reviewers: Blessed Reviewers, epriestley

Reviewed By: Blessed Reviewers, epriestley

Subscribers: Korvin, epriestley

Differential Revision: https://secure.phabricator.com/D18632