title: Api Platform DateTimeNormalizer not allowing null
body: <p>Because I prefer stackoverflow then github for such things, I am moving this <a href="https://github.com/api-platform/api-platform/issues/780" rel="nofollow noreferrer">issue</a> conversation here.</p>
  
  <p>For instance, you may have a user with <code>deletedAt</code> field. For some business reasons user is deleted but because of other reasons, you may want to activate him back by setting <code>deletedAt</code> field to be null. If you try to do it with API Platform, you are going to get this error:</p>
  
  <p><code>The data is either an empty string or null, you should pass a string that can be parsed with the passed format or a valid DateTime string.</code></p>
  
  <p>The question is, what is the best way to handle it?</p>