Add null check to AllowedMentions.ToModel() (#1865)

src/Discord.Net.Rest/Extensions/EntityExtensions.cs

@@ -71,6 +71,7 @@ namespace Discord.Rest
 
         public static API.AllowedMentions ToModel(this AllowedMentions entity)
         {
+            if (entity == null) return null;
             return new API.AllowedMentions()
             {
                 Parse = entity.AllowedTypes?.EnumerateMentionTypes().ToArray(),