PHP 8.4 fix code

[jaro3]

Current Nextcloud 31 php-fpm 8.4 on Apache error log polluted with this issue:

OCA\Passman\Service\CredentialService::getCredentialByGUID(): Implicitly marking parameter $user_id as nullable is deprecated, the explicit nullable type must be used instead at /usr/share/webapps/nextcloud/apps/passman/lib/Service/CredentialService.php#

The fix is described here:

bcit-ci/CodeIgniter#6302

https://dev.to/gromnan/fix-php-84-deprecation-implicitly-marking-parameter-as-nullable-is-deprecated-the-explicit-nullable-type-must-be-used-instead-5gp3