You can create something for all null values like below.
Say you have Category null.. then you can write something in your script..
IF(IsNull(Category) or Len(Trim(Category))=0,'N',Category) as Category
So now all Null Category will be replaced by 'N', which you can select at front end list box.
In the filter pane we cant see the Null values. If you have taken the field in table you can able to see the Null values present in your field.
Suppose if your Value field contains Null Values.
To convert Null(-) into 'N' . Need to use the below script before QVD. In my example I have considered '<NoData>'
you can replace with 'N'
after reloading the script, in the value field wherever we have Null(-) those will be converted to <NoData>.
If you have taken value field in the filter pane also you can find N in place of Null(-) and remaining records as it is.
Hope it helps
I think thats what manish suggested, if the value in the field is null or empty it will be replaced by 'N' or if the value is not null then the original value is placed in the field.
Are you getting only 'N' values ?