Issue in T-SQL while checking null variable

Question

I am writing a trigger in SQL Server like this

DECLARE @N_CATComp smallint, @EXON varchar(69), @DATEDEB varchar(15), @DATEFIN varchar(15)

SELECT 
    @N_CATComp = N_CATCompta, 
    @EXON =[Exon N°],
    @DATEDEB = CONVERT(varchar(15), [Date début] , 103), 
    @DATEDEB = CONVERT(varchar(15), [Date début] , 103) 
FROM INSERTED

IF  (@N_CATComp =4) AND  ( ISNULL(@EXON,'')='' OR ISNULL(@DATEDEB,'') ='' OR ISNULL(@DATEFIN,'') ='') 
BEGIN
     RAISERROR('false',11,1)
END

MY problem is that when @exon, @datedeb,@datefin are not null and catcomp=4 the raiserror appears which it shouldn't i tried to elpace isnull by for example len(@EXON)=0 in this case ven if the values are null then the raiserror don't appears

Answer 1

Here is the same thing with all those scalar variables removed.

IF EXISTS
(
    SELECT *
    from inserted
    where N_CATCompta = 4
        AND 
        (
            ISNULL([Exon N°], '') = '' 
            OR 
            ISNULL([Date début], '') = '' 
        ) 
)   
     RAISERROR('false',11,1)