Fix SocketGuild not returning the AudioClient (#2248)

src/Discord.Net.WebSocket/Entities/Guilds/SocketGuild.cs

@@ -1781,7 +1781,7 @@ namespace Discord.WebSocket
         /// <inheritdoc />
         ulong? IGuild.AFKChannelId => AFKChannelId;
         /// <inheritdoc />
-        IAudioClient IGuild.AudioClient => null;
+        IAudioClient IGuild.AudioClient => AudioClient;
         /// <inheritdoc />
         bool IGuild.Available => true;
         /// <inheritdoc />