title: Map Target object with no source object using MapStruct
body: <p>I want to map a <code>Target</code> object with no source object using MapStruct. I tried it, but getting the below  error.</p>
  
  <blockquote>
    <p>Can't generate mapping method with no input arguments</p>
  </blockquote>
  
  <p>Mapper code</p>
  
  <pre><code>public interface MyMapper {
  @Mapping(target="student.courseName", constant="Master in Science")
  Target map();
  }
  </code></pre>