T15883 model extend

[niden]

Hello!

• Type: bug fix
• Link to issue: [BUG]: Lib\\Models\\RobotError::findFirst(): Return value must be of type ?Phalcon\\Mvc\\ModelInterface, Phalcon\\Mvc\\Model\\Row returned #15883

In raising this pull request, I confirm the following:

• I have read and understood the Contributing Guidelines
• I have checked that another pull request for this purpose does not exist
• I wrote some tests for this PR
• I have updated the relevant CHANGELOG
• I have created a PR for the documentation about this change

Fixed Phalcon\Mvc\Model::findFirst() to return mixed or null

Thanks