avoid metadata persisting in header bar when missing from playing track

Summary: ensure that if artist, albumArtist or title are empty, they become empty in header bar

Test Plan: play a track with all metadata and next a track without album metadata

Reviewers: Elisa, astippich, ngraham, jguidon

Reviewed By: Elisa, astippich

Differential Revision: https://phabricator.kde.org/D24231