Compatibility with php 8.4

In the file **src/Exception.php, line 32** should be updated:

`public function __construct(Entity $entity, $message = "", $code = 0, \Exception $previous = null)`

to

`public function __construct(Entity $entity, $message = "", $code = 0, ?\Exception $previous = null)`

because Implicitly nullable parameter types are now deprecated in PHP 8.4.

Additionally, in the file **src/FluentLogger.php, line 111** should be changed from:

`PackerInterface $packer = null)`

to

`?PackerInterface $packer = null)`

for the same reason: implicitly nullable parameter types are deprecated in PHP 8.4.