add null to stringify type definition

[quangv]

Comparing the jsdoc type in index.js to types/index.d.ts seems like null is missing for stringify, 2nd argument.

* @param {((this: any, key: string, value: any) => any) | (string | number)[] | null | undefined} [replacer]

[WebReflection]

isn't that covered by replacer? ... or maybe not? 🤔 true that you can pass null, if you confirm that's an issue in TS I might change that and accept your PR, thanks!