Make $response param explicitly nullable

[JonPurvis]

Hey 👋

Whilst upgrading my application to PHP8.4, I was getting these deprecation warnings when running my test suite:

{closure:Spatie\Sitemap\SitemapGenerator::__construct():48}(): Implicitly marking parameter $response as nullable is deprecated, the explicit nullable type must be used instead

and

{closure:Spatie\Sitemap\SitemapGenerator::__construct():177}(): Implicitly marking parameter $response as nullable is deprecated, the explicit nullable type must be used instead

This PR ensures that the $reponse param is explicitly nullable and therefore no longer shows the deprecation warning.

Thanks!

[freekmurze]

Thanks!