fix(backend): make track media fields and external id nullable

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.

backend/Musify.Infrastructure/MassTransit/Logs/UpdateTrackPictureLog.cs

@@ -2,8 +2,8 @@ namespace Musify.Infrastructure.MassTransit.Logs
 {
     public record UpdateTrackPictureLog(
         Guid TrackId,
-        string PreviousOriginalPictureKey,
-    string? PreviousSmallPictureKey,
-    string? PreviousMediumPictureKey,
-    string? PreviousLargePictureKey);
+        string? PreviousOriginalPictureKey,
+        string? PreviousSmallPictureKey,
+        string? PreviousMediumPictureKey,
+        string? PreviousLargePictureKey);
 }