[nullsafety] constant object literals with nullable fields

Null safety is smart enough to detect that `a` can never be null here despite being declared as `Null`, so this compiles:

```haxe
var a:Null> = [];
a.push(0);
```

Consequently, this should also compile:

```haxe
@:nullSafety
class Main {
static function main() {
var s:Struct = {
values: []
}
s.values.push(0); // Null safety: Cannot access "push" of a nullable value.
}
}

typedef Struct = {
?values:Array
}
```