code: protected function getDefaultSerial($serial_number) {
          if (empty($serial_number)) {
              $devices_serials = $this->getDevices();
              if (count($devices_serials) == 0) {
                  $devices_serials = $this->getDevices(DEVICE_TYPE_PROTECT);
              }
              $serial_number = $devices_serials[0];
          }
          return $serial_number;
      }
docs: Either return the parameter as-is, or, if empty, return the serial number of the first device found in the user's account.
  
  @param string $serial_number Serial number will be returned, if specified.
  
  @return string Serial number of the first defined device.