Hide curtainview on mobile if it's empty

Summary: If we don't have any panels, just an action list, we want to hide the entire box on mobile since it's just an empty line.

Test Plan: Review Owners, Differential curtains on mobile, desktop.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D18350