Do not continue displaying the previous track's album art image when the new track's albumArtUrl is empty

Summary:
When a new track starts playing and does not have album art the previously displayed
album art image will be displayed until a new track starts playing that does have
album art

Test Plan:
Before applying this patch play a track that has album art followed by a song
that does not have album art. Notice that when the track without album art starts
playing the album art of the previous track is still displayed

Apply patch, repeat previous test and notice that the album art placeholder is displayed
when the track without album art start playing

Reviewers: #kde_connect, nicolasfella

Reviewed By: #kde_connect, nicolasfella

Subscribers: kdeconnect

Tags: #kde_connect

Differential Revision: https://phabricator.kde.org/D17875