src(WebSocket): fix race condition (#3636)

A race condition caused Client#user to be null in the ready event if the client handled 0 guilds.

src/client/websocket/WebSocketShard.js

@@ -373,7 +373,6 @@ class WebSocketShard extends EventEmitter {
         this.debug(`[READY] Session ${this.sessionID}.`);
         this.lastHeartbeatAcked = true;
         this.sendHeartbeat('ReadyHeartbeat');
-        this.checkReady();
         break;
       case WSEvents.RESUMED: {
         /**

src/client/websocket/handlers/READY.js

@@ -16,4 +16,6 @@ module.exports = (client, { d: data }, shard) => {
     guild.shardID = shard.id;
     client.guilds.add(guild);
   }
+
+  shard.checkReady();
 };