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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_service_url( self, block_identifier: BlockSpecification, service_hex_address: AddressHex, ) -> Optional[str]: """Gets the URL of a service by address. If does not exist return None"""

        result = self.proxy.contract.functions.urls(service_hex_address).call(
            block_identifier=block_identifier,
        )
        if result == '':
            return None
        return result