T15266 paginator group null

[niden]

Hello!

• Type: bug fix
• Link to issue: [BUG]: Paginator\Adapter\QueryBuilder total count for group by with NULL values #15266

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\Paginator\Adapter\QueryBuilder::paginate to correctly count when having a GROUP BY for null columns

Thanks