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.Domain/Entities/Track.cs

@@ -1,4 +1,4 @@
-using System.ComponentModel.DataAnnotations;
+using System.ComponentModel.DataAnnotations;
 using System.ComponentModel.DataAnnotations.Schema;
 using System.Diagnostics.CodeAnalysis;
 using Musify.Domain.Abstractions;
@@ -23,23 +23,20 @@ namespace Musify.Domain.Entities
         [Required]
         public int Duration { get; set; }
 
-        [Required]
         [MaxLength(200)]
-        public string Artist { get; set; } = string.Empty;
+        public string? Artist { get; set; }
 
         [Required]
         public TrackSource Source { get; set; } = TrackSource.Local;
 
-        [Required]
         [MaxLength(16)]
-        public string ExternalId { get; set; } = string.Empty;
+        public string? ExternalId { get; set; }
 
         [Required]
         public bool DownloadRequested { get; set; } = false;
 
-        [Required]
         [MaxLength(64)]
-        public required string OriginalPictureName { get; set; }
+        public string? OriginalPictureName { get; set; }
 
         [MaxLength(64)]
         public string? SmallPictureName { get; set; }
@@ -50,9 +47,8 @@ namespace Musify.Domain.Entities
         [MaxLength(64)]
         public string? LargePictureName { get; set; }
 
-        [Required]
         [MaxLength(64)]
-        public required string OriginalAudioName { get; set; }
+        public string? OriginalAudioName { get; set; }
 
         [MaxLength(64)]
         public string? AudioFolderName { get; set; }