[FIX]Removed Implicit marking parameter as nullable

[hauthorn]

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We've been using this package for a long while, and when upgrading to PHP 8.4, we noticed output like this when running tests:

PHP Deprecated:  EllipseSynergie\ApiResponse\AbstractResponse::withCollection(): Implicitly marking parameter $cursor as nullable is deprecated, the explicit nullable type must be used instead in [PROJECT_FOLDER]/vendor/ellipsesynergie/api-response/src/AbstractResponse.php on line 131
PHP Deprecated:  EllipseSynergie\ApiResponse\Contracts\Response::withItem(): Implicitly marking parameter $resourceKey as nullable is deprecated, the explicit nullable type must be used instead in [PROJECT_FOLDER]/vendor/ellipsesynergie/api-response/src/Contracts/Response.php on line 62

This PR should fix that. Marking paramaters as nullable is supported in PHP 8.1.

[hauthorn]

@maximebeaudoin - Let me know if this is at all interesting to you, and if there's something else I can do to make the review easier for you.