fix(types): joinedAt is nullable (#932)

src/types/members/guild_member_update.ts

@@ -11,7 +11,7 @@ export interface GuildMemberUpdate {
   /** Nickname of the user in the guild */
   nick?: string | null;
   /** When the user joined the guild */
-  joinedAt: string;
+  joinedAt: string | null;
   /** When the user starting boosting the guild */
   premiumSince?: string | null;
   /** Whether the user has not yet passed the guild's Membership Screening requirements */