Fix implicit nullable parameter deprecations for PHP 8.4

[nige-one]

PHP 8.4 deprecates implicitly nullable parameters (typed parameters with a null default but no explicit nullable type). Loading these classes on PHP 8.4 raises:

Deprecated Functionality: DEG\CustomReports\Model\GenericReportCollection::__construct(): Implicitly marking parameter $resourceConnection as nullable is deprecated, the explicit nullable type must be used instead

In Magento developer mode this deprecation is escalated to an exception by Magento\Framework\App\ErrorHandler, which breaks setup:di:compile (proxy/repository code generation reflects these constructors).

This PR adds the explicit ? nullable type to the four affected parameters:

• Model/CustomReport.php: $resource, $resourceCollection
• Model/GenericReportCollection.php: $connection, $resourceConnection

No behavior change — the explicit nullable type is identical to the previous implicit semantics and remains compatible with PHP 7.1+.

Related: #51