QV20 Filter the null values of column in set analysis

I have SCOTT.EMP table with us, and want to filter out null values .
the basic requirement is to filter the data using set analysis whose MGR is null.

Dimension - MGR
expression - SUM (SAL)

Now i want to remove null's from dimension of chart (i.e. sum= 5000 where MGR is null),
i can do in various ways but i want to use using "set analysis"


expression - sum({<MGR-={'=Len(Trim(MGR))=0'} >} [SAL])