Subsonic: Fix `getArtistInfo` with `includeNotPresent=1`
Subsonic: Fix `getArtistInfo` with `includeNotPresent=1`

Including similar artists not found from the user's library was broken
since adding type declarations in a092fa4d. This was because
AlbumBusinessLayer::countByArtist got called with null argument which
is now forbidden by the type declaration.

paulijar/music,owncloud/music,owncloud/music,paulijar/music,paulijar/music,paulijar/music,owncloud/music,owncloud/music,owncloud/music,paulijar/music