I would imagine the preferred answer to that question is yes, and it's a school that values activism and political involvement of some sort. Leaving it blank implies a no IMO - just like when people answer "don't know" to the family income question I am never shocked when I scroll down and see both parents are listed as having high income careers lol.
But if you're that worried about it - if you're not eligible to vote, I imagine that would be obvious from your demographic information and would not count against you. If you are, it's not that hard to just register for the sake of the question even if you don't plan to vote frankly (but you should!).