docs: def create_contact(self, email=None, first_name=None, last_name=None, phone_number=None):
          """
          Create a contant which is later passed to payment.
          """
          result = {}
          if email:
              result['email'] = email
          if first_name is not None:
              result['first_name'] = first_name
          if last_name is not None:
              result['last_name'] = last_name
          if phone_number is not None:
              result['phone_number'] = phone_number
          return result if len(result) > 0 else None