Fixed a nullref when an embed is not provided to SendMessage

src/Discord.Net.Rest/Entities/Channels/ChannelHelper.cs

@@ -152,7 +152,7 @@ namespace Discord.Rest
         public static async Task<RestUserMessage> SendMessageAsync(IMessageChannel channel, BaseDiscordClient client,
             string text, bool isTTS, Embed embed, RequestOptions options)
         {
-            var args = new CreateMessageParams(text) { IsTTS = isTTS, Embed = embed.ToModel() };
+            var args = new CreateMessageParams(text) { IsTTS = isTTS, Embed = embed?.ToModel() };
             var model = await client.ApiClient.CreateMessageAsync(channel.Id, args, options).ConfigureAwait(false);
             return RestUserMessage.Create(client, channel, client.CurrentUser, model);
         }