title: Firestore select where a field is null
body: <p>How can I select all docs that do not have a certain field? I have the following structure:</p>
  
  <pre><code>user = {
      firstName: 'blabla',
      lastName: 'bloblo',
      address: {
          city: 'xxxx',
          state: 'xxx'
      }
  }
  </code></pre>
  
  <p>I have tried comparing with null, but wasn't successful.</p>
  
  <pre><code>let db = firebase.firestore();  
  let querySnapshot = await db
      .collection("users")
      .where("address", "==", null)
      .get();
  </code></pre>