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.Controller/Entities/UserItemData.cs

@@ -1,5 +1,3 @@
-#nullable disable
-
 #pragma warning disable CS1591
 
 using System;
@@ -19,17 +17,11 @@ namespace MediaBrowser.Controller.Entities
         /// </summary>
         private double? _rating;
 
-        /// <summary>
-        /// Gets or sets the user id.
-        /// </summary>
-        /// <value>The user id.</value>
-        public Guid UserId { get; set; }
-
         /// <summary>
         /// 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 users 0-10 rating.