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>'.
Yes, if you put that expression on the Sort tab you only change the sorting. That's why the tab has the name Sort. Go to the General tab and replace the field Expense Type with <expression> and then enter the expression I posted above.