title: Set null as empty string value for input field
body: <p>I have an <code>input</code> field mapped to an entity in my controller with a <code>ngModel</code> 2-way binding:</p>
  
  <pre class="lang-html prettyprint-override"><code>&lt;input type="text" [(ngModel)]="entity.one_attribute" /&gt;
  </code></pre>
  
  <p>When I initialize my controller, I have this entity:</p>
  
  <pre class="lang-typescript prettyprint-override"><code>{ one_attribute: null }
  </code></pre>
  
  <p>If a user starts to fill in the field but does not submit the form immediately and empties the field, my entity is updated to become:</p>
  
  <pre class="lang-typescript prettyprint-override"><code>{ one_attribute: "" }
  </code></pre>
  
  <p>Is it possible to define that empty string should be changed to <code>null</code> automatically?</p>