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