Not Equal to Zero Or Grater Than 0 filter is not working in set analysis expression
Dealing with nulls is complicated in QlikView since very data source use them in a different way. Since a null value is a value that doesn't exist, usually there's no way to identify them in one chart and select all values instead of those.
Actually, if you search the Community looking for them, you will find very different solutions, some of them worked some didn't. What I do recommend you is trying to identify them in the load script, flag them to use this flag later on.
Say for example that you have one field "MonthlySalesPercentage" that may be null. You can do the following in your script: