title: Firebase: flutter want to get a null value
body: <p>I want to get data that is null in a Firebase database</p>
  
  <pre><code>stream: Firestore.instance
        .collection('resep')
        .where("email", isEqualTo: null)
        .snapshots(),
  </code></pre>
  
  <p>I want to get email data that is null</p>