Fixes unused creation of REST clients for DiscordShardedClient shards. (#2109)

* Init

* Remove unnecessary length check

* Swap out for any check

* Final; Check if parentclient was passed

src/Discord.Net.WebSocket/DiscordSocketClient.cs

@@ -152,6 +152,7 @@ namespace Discord.WebSocket
             LogGatewayIntentWarnings = config.LogGatewayIntentWarnings;
             HandlerTimeout = config.HandlerTimeout;
             State = new ClientState(0, 0);
+            if (shardedClient is null || parentClient is null)
                 Rest = new DiscordSocketRestClient(config, ApiClient);
             _heartbeatTimes = new ConcurrentQueue<long>();
             _gatewayIntents = config.GatewayIntents;