Safety: Cannot cast nullable value to not nullable type for abstracts

```haxe
class Bug2 {
    public function bug(?v : haxe.Int32) : Void {
        // Safety: Cannot cast nullable value to not nullable type.
        v == null ? "A" : "B";
    }
}
```

See https://github.com/restorer/haxe-safety-bugs/blob/master/safetybugs/Main.hx#L50 for working example.