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/Persistence/Database.cs

@@ -32,8 +32,7 @@ namespace Musify.Infrastructure.Persistence
 
             modelBuilder.Entity<Track>()
                 .HasIndex(track => new { track.Source, track.ExternalId })
-                .IsUnique()
-                .HasFilter("\"ExternalId\" <> ''");
+                .IsUnique();
 
             var playListProcessing = modelBuilder.Entity<PlayListProcessingState>();
             playListProcessing.HasKey(state => state.CorrelationId);