Fix sticker args

src/Discord.Net.Rest/DiscordRestApiClient.cs

@@ -745,7 +745,7 @@ namespace Discord.API
         {
             Preconditions.NotNull(args, nameof(args));
             Preconditions.NotEqual(channelId, 0, nameof(channelId));
-            if (!args.Embed.IsSpecified || args.Embed.Value == null)
+            if ((!args.Embed.IsSpecified || args.Embed.Value == null) && (!args.Stickers.IsSpecified || args.Stickers.Value == null || args.Stickers.Value.Length == 0))
                 Preconditions.NotNullOrEmpty(args.Content, nameof(args.Content));
 
             if (args.Content?.Length > DiscordConfig.MaxMessageSize)