Create calcualted field if(isnull(Mesure1)) then measure1 else 0 end
measure 1 is the metrci used in report .
Have you tried ifnull(expression1,expression2) where the first expression is used (e.g. sum(sales) if the return it (the expression) is NOT null and the second is returned if it is null - example Ifnull(sum(sales),0) will look at the the sum of sales - if it is Null it will return a 0, if it is not null it will return the sum
Let me know if this helps