title: Ignore null fields when DEserializing JSON with Gson or Jackson
body: <p>I know there's lots of questions about skipping fields with a null value when serializing objects to JSON. 
  I want to skip / ignore fields with null values when deserializing JSON to an object. </p>
  
  <p>Consider the class</p>
  
  <pre><code>public class User {
      Long id = 42L;
      String name = "John";
  }
  </code></pre>
  
  <p>and the JSON string </p>
  
  <pre><code>{"id":1,"name":null}
  </code></pre>
  
  <p>When doing </p>
  
  <pre><code>User user = gson.fromJson(json, User.class)
  </code></pre>
  
  <p>I want  <code>user.id</code> to be '1' and <code>user.name</code> to be 'John'. </p>
  
  <p>Is this possible with either Gson or Jackson in a general fashion (without special <code>TypeAdapter</code>s or similar)?</p>