Don't show large empty chat log if contact doesn't support it

Summary: Instead, show just a simple QLabel (that was shown before, but together with empty chat log)

Test Plan:
See pictures. It works.

Before: {F6291961}
After: {F6291964}

Reviewers: dvratil, davidedmundson

Reviewed By: davidedmundson

Differential Revision: https://phabricator.kde.org/D15817