Fix for PHP 8.4 (#227)

> Deprecated: Gt\CssXPath\Translator::preg_match_collated(): Implicitly marking parameter $transform as nullable is deprecated, the explicit nullable type must be used instead in ./phpgt/cssxpath/src/Translator.php on line 279

src/Translator.php

@@ -340,7 +340,7 @@ protected function convertSingleSelector(string $css):string {
  protected function preg_match_collated(
  string $regex,
  string $string,
- callable $transform = null
+ ?callable $transform = null
  ):array {
  preg_match_all(
  $regex,