False positive [level 3] Offset does not exist on array()

# Bug report

I get a false positive with the code snippet bellow :

```
Offset 'a' does not exist on array().  
```

### Code snippet that reproduces the problem

https://phpstan.org/r/124648b3-4f8a-4ce1-aba1-258dfb221d86

### Expected output

No warning are expected