Artist, ExternalId, OriginalPictureName and OriginalAudioName were
non-nullable with '' sentinels, which forced the unique (Source,
ExternalId) index to rely on a filtered "<> ''" clause. Make them
truly nullable and drop the empty-string sentinels, backfilling
existing rows to NULL. Also fixes a formatting regression
(`track != null&&`) introduced alongside the nullability change.
Playing a YouTube song now creates a lightweight track row (without queueing a download) and writes listening history; saving to a playlist queues the download via the new DownloadRequested flag.