Artists with no album shouldn't be listed in Artists page

Steps to reproduce

Import album associated to Various Artists and with tracks associated to specific artist.

In the Artists page you can see some artists with 0 album and empty cards. It is confusing and looks awkward:

Those artists should not be displayed or displayed in a different way.

Funkwhale version(s) affected: 0.19.1

All songs were uploaded/imported on previous releases of Funkwhale.