title: How to form Firestore query matching with undefined elements
body: <p>I'm trying to perform the following query with Firebase's Firestore:</p>
  
  <pre><code>async function getTicketNumbers(companyID) {
  
      var ticketNumbers = []
  
      const docs = await store
          .collection('ABC')
          .doc(companyID)
          .collection('DEF')
          .where('ticketStatus', 'in', ['scheduled', undefined])
          .get()
  
      if (docs) {
          docs.forEach(doc =&gt; {
              ticketNumbers.push(doc.id)
          })
      }
  
      return ticketNumbers
  }
  </code></pre>
  
  <p>The problem in my code is the <code>undefined</code> keyword. But how can I perform a query where <code>TicketStatus</code> is either <code>scheduled</code> or <code>undefined</code>? </p>
  
  <p>I also tried <code>null</code> and empty string <code>''</code>, what does not result in an error, but does not return additional documents.
  Any help is highly appreciated.</p>