Show a message indicating that no TB devices are connected when the list is empty
Summary: ...Otherwise the user might think that it's just broken.
Test Plan:
Before:
After:
Reviewers: VDG, Plasma, dvratil, mart
Reviewed By: VDG, Plasma, mart
Subscribers: plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D24715