false-positive: Offset does not exist on via isset

### Code snippet that reproduces the problem

https://phpstan.org/r/11fd63b2-c630-4d76-8546-4f004231e273

https://3v4l.org/kiLe9

### Expected output

```isset()``` can check if the array key exists, so that there should be no error on line 21, or?

