title: Firestore Security Rule - Validate incoming data length is greater than 2 but field is not required
body: <p>I know how to check if a field of an incoming data set is a string or not, but how do I do a conditional check if the field exists, then check if its a string:</p>
  
  <p>For example, "last name" is not a required field, but if it's supplied, then check if its length is greater than 2. I get an error if I just check the length rule generally -> is there a way to check the length only if last_name exists?</p>
  
  <p>Firestore Rule: 
  request.resource.data.last_name.length() > 0</p>