src/XQueue/Maileon/API/Transactions/ContactReference.php
function isEmpty() {
        $result = !isset($this->id) && !isset($this->external_id) && !isset($this->email);
        
        return $result;
    }
function isEmpty() {
        $result = !isset($this->id) && !isset($this->external_id) && !isset($this->email);
        
        return $result;
    }
Signals to the JSON serializer whether this object should be serialized

@return boolean