message: fix(types): joinedAt is nullable
diff: @@ -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 */
  