title: Set null to target using mastruct
body: <p>I am using mapstruct and I am wondering if there are any ways to set null value for some target properties. For example,</p>
  
  <pre><code>public class MySource {
      private String prop1;
      private String prop2;
  
      public MySource() {
          // Initialization.
      }
  
      // Getters - Setters.
  }
  
  public class MySourceDto {
      private String prop1;
      private String prop2;
  
      public MySourceDto() {
          // Initialization.
      }
  
      // Getters - Setters.
  }
  
  @Mapper
  public interface MySourceMapper {
  
      @Mappings({
          @Mapping(target = "prop1", propertyToSetNull = null)})
      public MySourceDto toView(MySource mySource);
  
  }
  </code></pre>
  
  <p>I would love the above source, dto and mapper to generate the below source code,</p>
  
  <pre><code>@Component
  public class MySourceMapperImpl implements MySourceMapper {
  
      @Override
      public MySourceDto toView(MySource mySource) {
          if ( mySource == null ) {
              return null;
          }
  
          MySourceDto mySourceDto = new MySourceDto();
  
          mySourceDto.setProp1( mySource.getId() );
          mySourceDto.setProp2( null );
  
          return mySourceDto;
      }
  }
  </code></pre>