title: Retrofit null\empty Date handling
body: <p>I've got Date field in my POJO and I use retrofit to parse JSON. In some cases returning value of this field can be empty or null. And I get error:</p>
  
  <pre><code>Caused by: java.text.ParseException: Unparseable date: "" (at offset 0)
  at java.text.DateFormat.parse(DateFormat.java:618)
  at com.google.gson.DefaultDateTypeAdapter.deserializeToDate(DefaultDateTypeAdapter.java:105)
  </code></pre>
  
  <p>Is there any smart way to handle this exception with retrofit?</p>