repIace bIank with nuII

I am trying to load data from flat file, where ever there is empty replace with NULL.  Below is the function.

ifthenelse(trim_code(ALTR.ACCEPT_CODE) = '', NULL, trim_code(ALTR.ACCEPT_CODE))
------------------- trim_code custom function -------
return nvl(ltrim_blanks_ext(rtrim_blanks_ext($ToTrim)),'');
-----------------------------------------

Tried removing the custom function too, But still it is showing up as Blank.please advice.

ifthenelse( (ALTR.ACCEPT_CODE) = '', NULL, (ALTR.ACCEPT_CODE))