contains_only_latin argument is _optional_ string

src/Core/Checkout/Cart/SmcCartValidator.php

@@ -10,7 +10,7 @@ use SmcAddressLatin\Core\Checkout\Cart\Error\ShippingAddressInvalidCharactersErr
 
 class SmcCartValidator implements CartValidatorInterface
 {
-    private static function contains_only_latin(string $input): bool {
+    private static function contains_only_latin(?string $input): bool {
         if ($input === null) {
                 return true;
         }