title: In Cloud Firestore rules - How do I check if a key is null
body: <p>In <strong>Cloud Firestore Rules</strong> - I have a document called <code>task</code> and I want to see if some data (<code>assignee</code> field) is null / don't exists.</p>
  
  <p>I've tried:</p>
  
  <ol>
  <li><code>resource.data.assignee == null</code> - Does not work (Error)</li>
  <li><code>!resource.data.hasAll(['assignee'])</code> - Does not work (Error)</li>
  </ol>
  
  <p>From the documentation - it states that this indeed creates an error:
  <code>
  // Error, key doesn't exist
  allow read: if resource.data.nonExistentKey == 'value';
  </code></p>