Fix: serialization error on thread creation timestamp. (#2188)

src/Discord.Net.Rest/API/Common/ThreadMetadata.cs

@@ -21,6 +21,6 @@ namespace Discord.API
         public Optional<bool> Invitable { get; set; }
 
         [JsonProperty("create_timestamp")]
-        public Optional<DateTimeOffset> CreatedAt { get; set; }
+        public Optional<DateTimeOffset?> CreatedAt { get; set; }
     }
 }

src/Discord.Net.WebSocket/Entities/Channels/SocketThreadChannel.cs

@@ -121,7 +121,7 @@ namespace Discord.WebSocket
         internal new static SocketThreadChannel Create(SocketGuild guild, ClientState state, Model model)
         {
             var parent = guild.GetChannel(model.CategoryId.Value);
-            var entity = new SocketThreadChannel(guild.Discord, guild, model.Id, parent, model.ThreadMetadata.GetValueOrDefault()?.CreatedAt.ToNullable());
+            var entity = new SocketThreadChannel(guild.Discord, guild, model.Id, parent, model.ThreadMetadata.GetValueOrDefault()?.CreatedAt.GetValueOrDefault(null));
             entity.Update(state, model);
             return entity;
         }