title: How to check whether a particular field exist in a particular Firestore document?
body: <p>Lets say I have a document. I know its <code>ID</code>. Now I want to know whether inside that document a particular field exist or not. And if not, I want to create it. Is it possible?</p>
  
  <p>PS: I don't want to put the field name with my own and put that value as <code>null</code>.</p>