feat: fix deprecation for PHP 8.4 (#187)

Co-authored-by: Christopher Georg <christopher.georg@sr-travel.de>

src/Formatter/MoneyFormatter.php

@@ -107,7 +107,7 @@ public function getCurrency(Money $money): Currency
         return $money->getCurrency();
     }
 
-    protected function getDefaultNumberFormatter(string $currencyCode, string $locale = null): \NumberFormatter
+    protected function getDefaultNumberFormatter(string $currencyCode, ?string $locale = null): \NumberFormatter
     {
         if (is_null($locale)) {
             $locale = \Locale::getDefault();