Fix PHPStan errors to make CI pipeline pass

• Fix PHPDoc variable placement in matomo_reporting_api.module
• Add static return type and PHPStan ignore for new static() in create() methods
• Change empty() check to strict null comparison for QueryFactoryInterface
• Change property visibility from public to protected in AccessDeniedTest
• Replace deprecated at() method with once() in ConfigHandlerTest
• Remove redundant condition check in ConfigHandlerTest

🤖 Generated with Claude Code

Co-Authored-By: Claude noreply@anthropic.com

Closes #3509595