Fix PHP 8.5 deprecations

[cedric-anne]

This will fix the following deprecation:

Using null as an array offset is deprecated, use an empty string instead
Stack trace:
#0 vendor/league/uri/Uri.php(372): Symfony\Bridge\PhpUnit\DeprecationErrorHandler->handleError(8192, 'Using null as a...', '/home/runner/wo...', 372)
#1 vendor/league/uri/Uri.php(227): League\Uri\Uri->formatPort(NULL)
#2 vendor/league/uri/Uri.php(391): League\Uri\Uri->__construct(NULL, NULL, Object(SensitiveParameterValue), NULL, NULL, 'variables.foo', NULL, NULL)
#3 src/Parser/StylesheetParser.php(1280): League\Uri\Uri::new('variables.foo')