refactor: update PreviewTokenModel properties to use nullable strings

Author: reeshika-h

Contentstack.Management.Core/Models/Token/PreviewTokenModel.cs

@@ -13,12 +13,12 @@ public class PreviewTokenModel
         /// The name of the preview token.
         /// </summary>
         [JsonPropertyName("name")]
-        public string Name { get; set; }
+        public string? Name { get; set; }
 
         /// <summary>
         /// The description of the preview token.
         /// </summary>
         [JsonPropertyName("description")]
-        public string Description { get; set; }
+        public string? Description { get; set; }
     }
 }