2-step mapping with generics does not work correctly.

Consider the following situation: (removed getter/setters to keep it short)

class Request {
    Nullable<ChildRequest> child;
}

class ChildRequest {
    String name;
}

class RequestDto {
    JsonNullable<ChildRequestDto> child;
}

class ChildRequestDto {
    String name;
}

with the following mapper:

@Mapper
public interface RequestMapper {

    RequestMapper INSTANCE = Mappers.getMapper( RequestMapper.class );

    Request map(RequestDto dto);

    default <T> Nullable<T> jsonNullableToNullable(JsonNullable<T> jsonNullable) {
        if ( jsonNullable.isPresent() ) {
            return Nullable.of( jsonNullable.get() );
        }
        return Nullable.undefined();
    }

    default JsonNullable<ChildRequest> mapJsonNullableChildren(JsonNullable<ChildRequestDto> dtos) {
        if (dtos.isPresent()) {
            return JsonNullable.of( mapChild( dtos.get() ) );
        } else {
            return JsonNullable.undefined();
        }
    }

    ChildRequest mapChild(ChildRequestDto dto);
}

This will not compile, but throw the following errors:

ERROR Issue2377Mapper.java:19 Nullable<ChildRequest> does not have an accessible constructor.
ERROR Issue2377Mapper.java:19 Can't map property "JsonNullable<ChildRequestDto> child" to "Nullable<ChildRequest> child". Consider to declare/implement a mapping method: "Nullable<ChildRequest> map(JsonNullable<ChildRequestDto> value)".

instead of generating the following code:

        request.setChild( jsonNullableToNullable( mapJsonNullableChildren( dto.getChild() ) ) );

Discovered during analysis of #2377 .