Collapse favorites sidebar when empty

Summary:
Users occasionally remove all the favorites with that goal
in mind, and while not showing the empty area might have
consequences if bug 383302 gets imlemented, I think it's
an alright suggestion.

BUG:383353

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D7321