code: def prepare_batch(self):
          
          
          for cert in self.certificates_to_issue:
              self.certificate_handler.validate_certificate(cert)
  
          self.merkle_tree.populate(self.get_certificate_generator())
          logging.info('here is the op_return_code data: %s', b2h(self.merkle_tree.get_blockchain_data()))
          return self.merkle_tree.get_blockchain_data()
docs: Propagates exception on failure
          :return: byte array to put on the blockchain