`null` value isn't retained on a ListBuilder field of Builder class that is marked @nullable on Built class

my code:
```dart
abstract class MyClass implements Built {
@nullable
@BuiltValueField(wireName: r'my_field')
BuiltList get myField;
}
```
is being generated to

```
class MyClassBuilder implements Builder {
ListBuilder _myField;
ListBuilder get myField =>
_$this._myField ??= new ListBuilder();
set myField(ListBuilder myField) =>
_$this._myField = myField;
}
```

When I set value of `myField` to `null`, I was expecting it to return `null` on get, but it returns empty list instead (because of how the getter above is generated)

Appreciate any help on this matter! Thanks