it shouldn't eat your credits if it finds 0 emails
only scenario I can think of is your list is full of catch-all domains and providers give you emails but Zerobounce consider all catch all as "too risky", could that be it?
you can check if that's the case by displaying the (by default hidden) Zerobounce column and check the actual answer
If so, switching the validation to something that supports catch-all validation like Findymail could help (I know you have your own ZB account so bummer ๐คท )
if that's not it then not sure why that is, curious too!