Fix PHP 8.4 implicit nullable parameter deprecation

[ludiaz]

When running on PHP 8.4, the following deprecation notice is triggered during cache warmup:

Acsiomatic\DeviceDetectorBundle\CacheWarmer\ProxyCacheWarmer::warmUp(): Implicitly marking parameter $buildDir as nullable is deprecated, the explicit nullable type must be used instead

This happens because PHP 8.4 no longer supports implicitly nullable parameter types.

Location: src/CacheWarmer/ProxyCacheWarmer.php

Suggested Fix: Add the explicit nullable type ? to the parameter.

Diff:

• public function warmUp(string $cacheDir, string $buildDir = null)

• public function warmUp(string $cacheDir, ?string $buildDir = null)