fix(date): allow getDateString maybe argument

Author: unrevised6419

src/date.ts

@@ -1,10 +1,12 @@
 import type { Tagged } from 'type-fest';
-import type { Maybe, Optional } from './types.js';
+import type { Maybe, Nullable, Optional } from './types.js';
 
 export type ValidDate = Tagged<Date, 'valid'>;
 
 export function getDateString(date: ValidDate): string;
-export function getDateString(date: Date): Optional<string>;
+export function getDateString(
+	date?: Maybe<Date | string | number>,
+): Optional<string>;
 export function getDateString(
 	date?: Maybe<Date | string | number>,
 ): Optional<string> {