How to set null value in calculated column in HANA

Hello,

I'm trying to create a few count distinct measures, so for some cases i would want to have null value for this dimension, to avoid the recond being counted.
So then when calculating this new dimension hana doens't let me set NULL value in the else.
Also important is to mention that i use column engine.

So does someone know how to set a null value for calculated column?