code: protected String getSender(final SmsSendRequestDto smsSendRequestDto) {
      if (StringUtils.isNotBlank(smsSendRequestDto.getSender())) {
        return smsSendRequestDto.getSender();
      }
      Validate.notEmpty(defaultSender, "defaultSender must not be null or blank");
      return defaultSender;
    }
docs: Returns the sender of the request. If no sender is specified the default sender will be
  returned.
  
  @param smsSendRequestDto the sms request
  @return the sender
  @throws IllegalArgumentException if no sender is specified at all