In the report builder I have a chart based on report data, and I'm trying to filter out all the results where a certain picklist value is not defined (null).

So in the Report filters I added the condition if Picklist_Value__c does not equal "" .  This had the desired effect in the report's chart as all non-defined picklist values were filtered out. 

However, when I go to the Dashboard, import the report (refreshing just to be sure) the data appears with the null values unfiltered.  I don't understand why it would not be the same as the diagram in the report, but maybe it has to do with the fact that there is a hyphen.  I.e. it says that hyphen (-) has 21 results, valueA 13, valueB 6 etc... so it is not passing the filter.

Obviously I can't filter with a hyphen because it isn't a valid picklist value.  How do I prevent null results from appearing in my Dashboard?