by using Alt function u can do that.
let's say u have Measure as Sales and it contains null then simply use alt function like below
alt( sum(Sales),0) // this will replace null as 0
try to set null as value in back end script
Take care about Alt() function, cause it works like this:
The alt function is often used with number or date interpretation functions. This way, Qlik Sense can test different date formats in a prioritized order. It can also be used to handle NULL values in numerical expressions.
What it means? If you have a field with non numeric values, maybe Alt function will not solve your problem.