fix broken artists node in musicvideos

fixes this:
DEBUG: ExecuteXBMCAction : Translating ActivateWindow(Videos,MusicVideoArtists,return)
DEBUG: ExecuteXBMCAction : To ActivateWindow(Videos,MusicVideoArtists,return)
DEBUG: CGUIMediaWindow::GetDirectory (videodb://musicvideos/artist/)
DEBUG:   ParentPath = [videodb://musicvideos/artist/]
ERROR: GetDirectory - Error getting videodb://musicvideos/artist/
ERROR: CGUIMediaWindow::GetDirectory(videodb://musicvideos/artist/) failed