Purge empty albums/artists

model/album.go

@@ -38,4 +38,5 @@ type AlbumRepository interface {
 	GetStarred(...QueryOptions) (Albums, error)
 	Search(q string, offset int, size int) (Albums, error)
 	Refresh(ids ...string) error
+	PurgeEmpty() error
 }

model/artist.go

@@ -22,4 +22,5 @@ type ArtistRepository interface {
 	Search(q string, offset int, size int) (Artists, error)
 	Refresh(ids ...string) error
 	GetIndex() (ArtistIndexes, error)
+	PurgeEmpty() error
 }