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.Application/Tracks/Responses/TrackApplicationResponse.cs

@@ -6,9 +6,9 @@ namespace Musify.Application.Tracks.Responses
     public record TrackApplicationResponse(
         Guid Id,
         string Title,
-        string Artist,
+        string? Artist,
         TrackSource Source,
-        string ExternalId,
+        string? ExternalId,
         ProcessingStatus AudioStatus,
         int Duration,
         int ListensCount,