Explicit nullable on User array for InMemory

[tsluyter]

I am building a testing environment with Slim-Skeleton, on PHP 8.3.

While running composer test I noticed a number of deprecation warnings.

Deprecated: App\Infrastructure\Persistence\User\InMemoryUserRepository::__construct(): Implicitly marking parameter $users as nullable is deprecated, the explicit nullable type must be used instead in /var/www/src/Infrastructure/Persistence/User/InMemoryUserRepository.php on line 21

... which I've fixed by following the instructions from the deprecation warning.

Warning:       Your XML configuration validates against a deprecated schema.
Suggestion:    Migrate your XML configuration using "--migrate-configuration"!

... which was also fixed by following instructions.

[tsluyter]

It seems that my change to the phpunit.xml file is a duplicate for #336