title: ModelMapper: how to handle null values for enums
body: <p>I am trying to map one object type to another using ModelMapper.
  I have defined the following mapping in my PropertyMap implementation:</p>
  
  <pre><code>map().getExtended().setIncludeMaskType(MaskType.fromValue(source.getExtendedPollingType().getMaskSettings().getIncludeMask().getMaskType()));
  </code></pre>
  
  <p>The problem is that the source type method call <code>source.getExtendedPollingType().getMaskSettings().getIncludeMask().getMaskType()</code>
  returns String while the destination type accepts a <code>MaskType</code> enum</p>
  
  <p>Therefore I am using <code>MaskType.fromValue()</code> to convert the String to enum. 
  The problem is that value of fails with <code>Caused by: java.lang.IllegalArgumentException</code> since this is source method call basically returns null during configuration.</p>
  
  <p>So how should I handle this use case?</p>