fix/refactor: fix GuildMember#presence getter and cleanup (#2751)

* Fix guild being a guild and not an AbstractHandler in PresenceUpdate

* update the default guild to be a Guild, and not GuildMember

* getters return null instead of undefined

* fix lint

src/structures/GuildMember.js

@@ -124,7 +124,7 @@ class GuildMember extends Base {
       user: {
         id: this.id,
       },
-      guild: this,
+      guild: this.guild,
     });
   }