Can you attach your workbook? This way I will be able to try different things.
Otherwise, at first thought what I'd do is to:
1) Create a T/F field like if ZN(CNT[Stk Nbr])=0 then TRUE else FALSE.
2) Add the field to size or colour. Then go to the legend on the right and right click on the TRUE and select hide.
Hopefully this should do the trick. If does help, please mark it as helpful and correct so other users can refer to it. Many thanks!