Add type-hint for getMonthFormat function

Add type information to the `getMonthFormat` function.

Bug: T384867
Change-Id: I5a5441458f456f15a4b2d2020a8f4612927942a6

lib/includes/Formatters/MwTimeIsoFormatter.php

@@ -150,7 +150,7 @@ private function getDayFormat( $dateFormat ) {
 	 *
 	 * @return string A date format for the month that roundtrips the Wikibase TimeParsers.
 	 */
-	private function getMonthFormat( $dateFormat ) {
+	private function getMonthFormat( ?string $dateFormat ): string {
 		if ( $dateFormat && preg_match( '/(?:[FMn]|(?<!x)m|xg)[.,]?/', $dateFormat, $matches ) ) {
 			return $matches[0];
 		}