fix: make nullable parameter types explicit for PHP 8.1+ compatibility

[ihxnnxs]

When running the application on PHP 8.1+, the following deprecation warnings appear:

PHP Deprecated:  Butschster\Head\MetaTags\Concerns\ManageTitle::setTitle(): Implicitly marking parameter $maxLength as nullable is deprecated, the explicit nullable type must be used instead
PHP Deprecated:  Butschster\Head\Contracts\MetaTags\MetaInterface::setTitle(): Implicitly marking parameter $maxLength as nullable is deprecated, the explicit nullable type must be used instead
PHP Deprecated:  Butschster\Head\Packages\Manager::create(): Implicitly marking parameter $callback as nullable is deprecated, the explicit nullable type must be used instead

These warnings are caused by parameters that are nullable (= null) but not explicitly typed with a nullable type (?type). This PR resolves the warnings by explicitly declaring the types as nullable, ensuring compatibility with PHP 8.1+ while remaining backward-compatible with PHP 8.0.