title: Firestore query filter retrieve document where field is missing
body: <p>Hello Firestore lovers !</p>
  
  <p>I am trying to retrieve all documents that do not have a field set. Here my field is named "source".</p>
  
  <ul>
  <li>I can get the documents where "source" is equal "".</li>
  <li>I can also get the documents where "source" is null.</li>
  <li>But I can't find the way to get documents where source is not set at all in the document.</li>
  </ul>
  
  <p>Sample python 3 working code using the google cloud python library :</p>
  
  <pre><code>from google.cloud import firestore
  
  product_ref = "product"
  db = firestore.Client()
  doc_ref = db.collection(product_ref)
  
  docs = doc_ref.where(u'source', u'==', None).get()
  </code></pre>
  
  <p>This works and returns all the document with source == null.</p>
  
  <p>I have found very few lines on github on a IS_NAN operator but I have not been able to use it to get the documents where "source" field is missing.</p>
  
  <p>Thank you !</p>