wow, i think for sure dont leave it blank, in my eyes that is the best question that can be asked for an applicant to toot his own horn. like someone said, theres far more to diversity than skin color, its a shame that the system has made us see it as only that. the way i interpret the question is: if every applicant including you had the same application and credentials, what then would make you unique and valuable to our community here? and maybe some obligatory commentary on why it would make you a good med student, doc and etc.
im certain that there are interesting facets about you that others enjoy experiencing in your presence. if you need to know what to write, consider even asking people you know what makes them value their association with you. blank, totally UNacceptable!