T17041 row offsetget

[niden]

Hello!

• Type: bug fix
• Link to issue: [BUG]: Phalcon\Mvc\Model\Row::offsetGet() throws on null column values in 5.13.0, was returning null in 5.12.1 and earlier #17041

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\Row::offsetGet() / offsetExists() throwing The index does not exist in the row when accessing a column whose value is null

Fixed Phalcon\Mvc\View::getContent() throwing TypeError after View::start(). start() was assigning $this->content = null

Thanks