Enable nullable for UserItemData

MetadataResult.GetOrAddUserData doesn't ever get used and is probably broken since the migration to .NET Core as it still expects a Guid for userId

MediaBrowser.Model/Dto/UserItemDataDto.cs

@@ -1,4 +1,3 @@
-#nullable disable
 using System;
 
 namespace MediaBrowser.Model.Dto
@@ -66,12 +65,12 @@ namespace MediaBrowser.Model.Dto
         /// Gets or sets the key.
         /// </summary>
         /// <value>The key.</value>
-        public string Key { get; set; }
+        public required string Key { get; set; }
 
         /// <summary>
         /// Gets or sets the item identifier.
         /// </summary>
         /// <value>The item identifier.</value>
-        public string ItemId { get; set; }
+        public Guid ItemId { get; set; }
     }
 }

MediaBrowser.Model/Session/UserDataChangeInfo.cs

@@ -1,4 +1,4 @@
-#nullable disable
+using System;
 using MediaBrowser.Model.Dto;
 
 namespace MediaBrowser.Model.Session
@@ -12,12 +12,12 @@ namespace MediaBrowser.Model.Session
         /// Gets or sets the user id.
         /// </summary>
         /// <value>The user id.</value>
-        public string UserId { get; set; }
+        public Guid UserId { get; set; }
 
         /// <summary>
         /// Gets or sets the user data list.
         /// </summary>
         /// <value>The user data list.</value>
-        public UserItemDataDto[] UserDataList { get; set; }
+        public required UserItemDataDto[] UserDataList { get; set; }
     }
 }