Change LastMessageId to nullable

src/Discord.Net/API/Common/Channel.cs

@@ -10,7 +10,7 @@ namespace Discord.API
         [JsonProperty("is_private")]
         public bool IsPrivate { get; set; }
         [JsonProperty("last_message_id")]
-        public ulong LastMessageId { get; set; }
+        public ulong? LastMessageId { get; set; }
 
         //GuildChannel
         [JsonProperty("guild_id")]

src/Discord.Net/API/Common/ReadState.cs

@@ -9,6 +9,6 @@ namespace Discord.API
         [JsonProperty("mention_count")]
         public int MentionCount { get; set; }
         [JsonProperty("last_message_id")]
-        public ulong LastMentionId { get; set; }
+        public ulong? LastMessageId { get; set; }
     }
 }