Don't report "is always true/false" for conditionally defined variables

[staabm]

closes https://github.com/phpstan/phpstan/issues/6830

[phpstan-bot]

This pull request has been marked as ready for review.

[ondrejmirtes]

The bug here is that PHPStan complains about "maybe undefined variable". Which it does only in foreach: https://phpstan.org/r/f934106d-8765-4463-8344-40b1c64ffc37