Altering built-in functions cast

Anyone know how I could alter the cast of the nullif()
function directly to return INT?

We use NULLIF() for adding [1|0|null] according to the
evalution of nullif('x','') into integer columns.
Where x is an integer and sometimes a empty string,
which if it's an empty string (x='') then we add NULL
cause NULLIF says if '' == '' then return NULL

Thank you,
matthew

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