Offset string on array(...) in isset() does not exist.

# Bug report

phpstan version: 0.12.18

### Code snippet that reproduces the problem

https://phpstan.org/r/6d243e33-295a-4a1a-9935-08ecc83ed484

Error message:

`Offset string on array(21021200 => '21028800') in isset() does not exist.`

### Expected output

No error.

Similar issue: #1062