fix: toISODate, toISO and toSQLDate methods could return null when an InvalidDate is provided

[edoardomistretta]

toISODate method could return null even if it is not explicit in the method type.

import { DateTime } from "luxon";

const invalidDate = new Date('2023-15-56'); // -> InvalidDate
const dt = DateTime.fromJSDate(invalidDate) // -> DateTime
const isoDt = dt.toISODate(); // -> null

A pull request for DefinitelyTyped is already opened in DefinitelyTyped/DefinitelyTyped#64919

[linux-foundation-easycla]

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