CP-17568 simple current

[niden]

Hello!

• Type: bug fix
• Link to issue: [BUG]: Resultset\Simple::current() returns false, which its declared return type forbids #17568

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

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Phalcon\Mvc\Model\Resultset\Simple::current() giving back false on a second call at a position with no row; the false it caches is a sentinel, and null is returned as the declared type says.

Thanks