fix(Message): fix thread getter (#6309)

For ThreadChannel, errors occur because channel.threads do not exist.

src/structures/Message.js

@@ -410,7 +410,7 @@ class Message extends Base {
    * @readonly
    */
   get thread() {
-    return this.channel?.threads.resolve(this.id) ?? null;
+    return this.channel?.threads?.resolve(this.id) ?? null;
   }
 
   /**