Fix php8.4 deprecated notices

Fixes ``` PHP Deprecated: Lab404\Impersonate\Exceptions\MissingUserProvider::__construct(): Implicitly marking parameter $previous as nullable is deprecated, the explicit nullable type must be used instead in ./src/Exceptions/MissingUserProvider.php on line 9 PHP Deprecated: Lab404\Impersonate\Exceptions\InvalidUserProvider::__construct(): Implicitly marking parameter $previous as nullable is deprecated, the explicit nullable type must be used instead in ./src/Exceptions/InvalidUserProvider.php on line 9 ```