Apply source presence tracking recursively in nested source #1456

public class SourceA {
  private String fieldA;
  private SourceB fieldB;

//getters & setters & hasXXX methods
...
}

public class SourceB {
  private String fieldC;
//getters & setters & hasXXX methods
...
}

//target
public class TargetObj {
  private String fieldA;
  private String fieldC;
//getters & setters
...
}

@Mapper(nullValueCheckStrategy =  NullValueCheckStrategy.ALWAYS)
public interface TestMapper {

  @Mapping(source = "fieldB.fieldC", target = "fieldC")
  TargetObj testMap(SourceA source);
}

public TargetObj testMap(SourceA source) {
    if(source == null) {
      return null;
    } else {
      TargetObj targetObj = new TargetObj();
      String fieldC = this.sourceFieldBFieldC(source);
      if(source.hasFieldB()) {
        targetObj.setFieldC(fieldC);
      }

      if(source.hasFieldA()) {
        targetObj.setFieldA(source.getFieldA());
      }

      return targetObj;
    }
  }
...

if(source.hasFieldB()) {
        targetObj.setFieldC(fieldC);
      }

if(source.hasFieldB() && source.getFieldB().hasFieldC()) {
        targetObj.setFieldC(fieldC);
      }