title: Firestore security rule that only allows empty documents
body: <p>I'm basically trying to use a firestore collection as a an email list. Anyone can create a document that has their email as the id and nothing more. The tricky part is the &quot;and nothing more&quot; bit. When no data is provided in the request, <code>request.resource</code> is <code>undefined</code> which you can't check for in security rules to my knowledge. Is this possible? Or is it necessary to have something like one mandatory field for this use case?</p>