title: How to query Cloud Firestore for non-existing keys of documents
body: <p>Let's say I have a data model with some optional properties. This could be for example a user object with a "firstname", a "lastname" and an optional "website" property. </p>
  
  <p>In Cloud Firestore only user documents with a known website would have the "website" property set, for all other user documents this property would not exist.</p>
  
  <p>My questions is now how to query for all user documents <strong>without</strong> a "website" property. </p>
  
  <p>Thanks.</p>