fix browsing albums from an artist and a genre but without album artist

Summary:
null album artist case needs to be handled to extract all albums from artist and genre

BUG: 411300

Test Plan: new automatic test is KO without patch and OK with it

Reviewers: Elisa, astippich, ngraham

Reviewed By: Elisa, ngraham

Differential Revision: https://phabricator.kde.org/D23720