code: def get_service_url(
              self,
              block_identifier: BlockSpecification,
              service_hex_address: AddressHex,
      ) -> Optional[str]:
          
          result = self.proxy.contract.functions.urls(service_hex_address).call(
              block_identifier=block_identifier,
          )
          if result == '':
              return None
          return result
docs: Gets the URL of a service by address. If does not exist return None