fix: toISODate, toISO and toSQLDate methods could return null when an InvalidDate is provided (#1406)

Author: edoeel

src/datetime.js

@@ -1823,7 +1823,7 @@ export default class DateTime {
    * @example DateTime.now().toISO() //=> '2017-04-22T20:47:05.335-04:00'
    * @example DateTime.now().toISO({ includeOffset: false }) //=> '2017-04-22T20:47:05.335'
    * @example DateTime.now().toISO({ format: 'basic' }) //=> '20170422T204705.335-0400'
-   * @return {string}
+   * @return {string|null}
    */
   toISO({
     format = "extended",
@@ -1850,7 +1850,7 @@ export default class DateTime {
    * @param {string} [opts.format='extended'] - choose between the basic and extended format
    * @example DateTime.utc(1982, 5, 25).toISODate() //=> '1982-05-25'
    * @example DateTime.utc(1982, 5, 25).toISODate({ format: 'basic' }) //=> '19820525'
-   * @return {string}
+   * @return {string|null}
    */
   toISODate({ format = "extended" } = {}) {
     if (!this.isValid) {
@@ -1935,7 +1935,7 @@ export default class DateTime {
   /**
    * Returns a string representation of this DateTime appropriate for use in SQL Date
    * @example DateTime.utc(2014, 7, 13).toSQLDate() //=> '2014-07-13'
-   * @return {string}
+   * @return {string|null}
    */
   toSQLDate() {
     if (!this.isValid) {