Disable shadows when no artist image is present

Summary:
This patch disables the shadow effect in the artists section when it shows the artist icon.
Just an aesthetic change, I think Breeze's monochromatic icons look better without shadows.

Before:

After:

Reviewers: Elisa, mgallien

Reviewed By: Elisa, mgallien

Subscribers: andreask, mgallien, astippich

Tags: VDG

Differential Revision: https://phabricator.kde.org/D9265