I had a similar problem after my iOS update. My back up wouldn’t complete, it showed up as 0 Kb, the next back up showed up as 0Kb and I didn’t get an option to delete my existing back up.
I turned back up off and whilst I didn’t then get an option to delete my previous back up, the previous back up (0 Kb) did disappear of it’s own accord. I was then able to turn back up back on, the 0 Kb back up did not reappear and I was able to start and complete a new back up.
Some have reported that after they turn back off, the previous back up (0 Kb) does not disappear or that if it does it reappears when back up is turned back on (although they are then able to make a new back up providing they have space).
It seems that in some case the 0Kb back up is corrupted data and does indeed take up space in the cloud and it doesn’t appear that this can be removed by the user, if this is what has happened in your case, contact Apple Support, (change country if necessary) they have been able to remove the corrupted data for some users.