fix nullability

MediaBrowser.Model/Providers/ExternalIdInfo.cs

@@ -9,13 +9,13 @@ namespace MediaBrowser.Model.Providers
         /// Gets or sets the display name of the external id provider (IE: IMDB, MusicBrainz, etc).
         /// </summary>
         // TODO: This should be renamed to ProviderName
-        public string Name { get; set; }
+        public string? Name { get; set; }
 
         /// <summary>
         /// Gets or sets the unique key for this id. This key should be unique across all providers.
         /// </summary>
         // TODO: This property is not actually unique across the concrete types at the moment. It should be updated to be unique.
-        public string Key { get; set; }
+        public string? Key { get; set; }
 
         /// <summary>
         /// Gets or sets the specific media type for this id. This is used to distinguish between the different
@@ -31,6 +31,6 @@ namespace MediaBrowser.Model.Providers
         /// <summary>
         /// Gets or sets the URL format string.
         /// </summary>
-        public string UrlFormatString { get; set; }
+        public string? UrlFormatString { get; set; }
     }
 }