There isn't any citric acid in Coke Zero whereas there is in diet Coke. At Xmas my LL counsellor said she couldn't actually say from an LL point of view that it was ok to drink it but it probably was.
I think I only survived not giving in and eating over Xmas because I had a pint of Coke Zero a day but I am one of the lucky ones and it hasn't brought me out of ketosis. However I have been in ketosis for 17 weeks so its firmly established. So take all the advice above, you may be one of the lucky ones, you may not. But if you are in your early days it isn't worth bringing yourself out of ketosis for in my opinion.