offset does not exist with isset and array shape

### Discussed in https://github.com/phpstan/phpstan/discussions/6507

<div type='discussions-op-text'>

<sup>Originally posted by **franmomu** February  1, 2022</sup>
Hi, is there a way to let know PHPStan about the existing offset?

https://phpstan.org/r/5a567579-444d-4ced-95a0-9c04004dab94

thanks!</div>