It's likely not missing the concept of zero but rather the symbol in the numbering system. The 0 is not necessary for representing integers (if there's a new symbol for 10). I'm pretty sure it is necessary to write the decimal part of a real number.
It's a fascinating problem. It makes me wonder, without zero, what paths mathematics would have taken. There have been civilisations that used math extensively without zero and I hope Ithkuil-fluent mathematicians some day would continue exploring this.
"Zero: The Bibliography of a dangerous idea" was quite a memorable book for me, and goes into some good detail on those very points - even trying to guesstimate just how damage to progress of civilization by the numerous rejections of zero at several points in our early history. http://books.google.com.au/books/about/Zero.html?id=obJ70nxV...