[null safety] False positive when passing `{a:Int}` to `{}`

[tobil4sk]

@:nullSafety
function main() {
  final other:{a:Int} = {a: 10};
  final data:{} = other; // ok
    
  final data:{} = {a: 10}; // Null safety: Cannot assign nullable value here.
}

With null safety disabled it unifies fine. Not a recent regression, the error goes back at least to 4.0.5.

https://try.haxe.org/#79528e3a