title: MongoDB - how to only update field if field does not exist
body: <p>How can I update a mongo document with the following requirements:</p>
  
  <p>Find a document by email property:</p>
  
  <p>If the document exists:</p>
  
  <ol>
  <li>If both retrieved and new document have property A, keep property A (the retrieved one).</li>
  <li>If retrieved document property A is null or undefined or doesn't exist, update using property A of the new object.</li>
  </ol>
  
  <p>If the document doesn't exist</p>
  
  <ol start="3">
  <li>Insert the new document.</li>
  </ol>
  
  <p>The <code>findOneAndUpdate</code> seems not to convey the both 3 of the requirements. Thanks.</p>