Don't put a spacer line on the bottom of previews

Summary: Fixes T8182. See screenshot in that task. We currently render a line to nowhere at the bottom of these previews. Instead, only render a line at the top.

Test Plan:
{F409078}

Also looked at a couple other applications that use this and they looked correct.

Reviewers: btrahan, lpriestley, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T8182

Differential Revision: https://secure.phabricator.com/D12905