[php][bug] Fix offsetGet in model generic to return offset rather than bool|null

[alsma]

Reverted previous behavior for offsetGet method

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@jebentier (2017/07), @dkarlovi (2017/07), @mandrean (2017/08), @jfastnacht (2017/09), @ackintosh (2017/09) ❤️, @ybelenko (2018/07), @renepardon (2018/12)

[jimschubert]

Thanks for the bug fix!

Usually I'd wait for template maintainer responses before merge. It's been ~7 years since I last used PHP, but I think the fix here is pretty obvious. If one of the template maintainers has any concerns, please open another issue.