fix: GuildEmbed.ChannelId as nullable per API documentation (#1532)

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

@@ -1,4 +1,4 @@
-#pragma warning disable CS1591
+#pragma warning disable CS1591
 using Newtonsoft.Json;
 
 namespace Discord.API
@@ -8,6 +8,6 @@ namespace Discord.API
         [JsonProperty("enabled")]
         public bool Enabled { get; set; }
         [JsonProperty("channel_id")]
-        public ulong ChannelId { get; set; }
+        public ulong? ChannelId { get; set; }
     }
 }