QlikView should not display Null values in the list box selection. Could you try right clicking and exporting to excel to see if it is a space
You could fix this in the loadscript with something like : if(Field = ' ', Null(), Field) as Field
A better solution would be to fix it in the datasource
That's not a null value, but an empty string or a value of one or more spaces. Listboxes don't show nulls. If you want to hide it then you need to use an expressions like =if(Len(Trim([Expense Type]))>0,[Expense Type])
A better solution is to replace that value in the script with a value like '<N/A>' or '<missing>'.