This question is truly optional, and I have not seen an applicant hurt (or helped) by not answering this. Personally, I would just answer the question.. It's not meant to be complicated. If someone doesn't answer, I assume that they're either white or Asian.
We also get to see everyone in-person on the interview day, so schools will eventually know every applicants' race unless someone plans on going to the interview in a hazmat suit or full body costume a la Mysterio (nb: neither is recommended).