Null Value in Dependency Calculation

We have a business scenario which requires us to include the variable of null (not zero, but blank) as a dependency in our classification characteristic

dependency.

That is, we have the following:

$SELF.variable A=

330

if variable B= '10'

AND (variable C = ????)

AND ((variable D LT 50

AND

Variable E LT 6) OR

Variable E LT 160)

Variable C needs to be a

null value for the dependency. We have tried two single quotes to represent blank and two double quotes to represent blank (e.g., ""), and tried the same with a space in between the quotes but we are getting syntax errors (E28024) with this entry.

Any ideas how to manage this scenario?