sample size calculation for paired proportions when one discordant pair is 0

#1
Hello!

I am trying to calculate sample size for a study using paired data. My groups are such that one discordant pair will always be 0. I planned to use McNemar to test for a difference between the groups, however, I am not sure this is valid since one discordant pair will always be zero. Is there a better way to analyze and therefore I should change this sample size calculation?

Thanks!