Orika no mapping of null elements in list

I have the following class:

public class A{
    List<AA> aaList;

    public A(List<AA> aaList){
        this.aaList = aaList;
    }

    //getters and setters + default constructor

    public class AA {
        String aaString;
        public AA(String aaString){
            this.aaString = aaString;
        }

        //getters and setters + default constructor
    }
}

And I want to have two objects of the same class, let's say:

A a = new A(Arrays.asList(new A.AA(null)));
A a2 = new A(Arrays.asList(new A.AA("test")));

and when I map a to a2, a2 should remain test because a has a null.

How can I configure this with Orika?

I tried something like:

mapperFactory.classMap(A.AA.class, A.AA.class)
            .mapNulls(false)
            .byDefault()
            .register();

    mapperFactory.classMap(A.class, A.class)
            .mapNulls(false)
            .customize(new CustomMapper<A, A>() {
                @Override public void mapAtoB(A a, A a2,
                        MappingContext context) {
                    map(a.getAAList(), a2.getAAList());
                }
            })
            .byDefault()
            .register();

Thanks in advance