Error in my stored procedure

[Rodrigo Reis]

I've been created this SP:


DELIMITER $$

DROP PROCEDURE IF EXISTS `ml_dados`.`validate_ticket` $$
CREATE DEFINER=`root`@`localhost` PROCEDURE `validate_ticket`(

OUT PAR_Resposta INTEGER,
IN PAR_EmailDoConvidado VARCHAR(100),
IN PAR_CodigoDoConvite VARCHAR(32)

)
BEGIN

DECLARE VAR_Res INTEGER;

SET VAR_Res = NULL;

SELECT Chave
FROM convites
WHERE EmailDoConvidado = PAR_EmailDoConvidado
AND CodigoDoConvite = PAR_CodigoDoConvite
INTO VAR_Res;

CASE
WHEN VAR_Res IS NULL THEN SELECT 'FALSO' AS Situacao;
ELSE SELECT 'VERDADEIRO' AS Situacao;
END CASE;




END $$

DELIMITER ;





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But, i need that out parameter retunr null when the select above
do not return any result, but at this case the SP do not return
nothing because "NO DATA - ZERO ROW FETCHED..." there is any solution
for my problem??