[null-safety] final vs. constructor throw

```haxe
class C {
final s:String;

public function new() {
if (Math.random() > 0.5) {
throw "no";
} else {
s = "foo";
}
}
}
```

`Null safety: Field "s" is not nullable thus should have an initial value or should be initialized in constructor.`

This is not accurate because _if_ we get an object from the constructor the field will be assigned to.