`mixed` type is breaking conditional analysis type narrowing

# Bug report

Analysis must be consistent, variable type before replacement must not be decisive.

### Code snippet that reproduces the problem

https://phpstan.org/r/a323a83a-107e-401a-bd61-6c696d29dccb

### Expected output

```diff
 Line | Error
 -- | --
-16 | Dumped type: mixed
+16 | Dumped type: null
 31 | Dumped type: null
```