Dynamic array key access does not work if key is optional even after isset()

# Bug report

This shows it best.

https://phpstan.org/r/5bd531ea-0a41-4be8-bbf9-3ca42e8d0bbe

It seems to be a "regression" of the 1.5.4 release which is unsurprising given it fixed a lot of issues related to arrays. But I do believe the behavior is incorrect and hope it can be fixed.

### Expected output

No error