What should I write in a personal history statement (diversity statement) if I am not at all diverse?
Concisely and honestly.

You'll look like an idiot if you cite your love of [insert minority] or your [questionable claim] heritage or your vast experience with [sympathetic cause]. 

Instead, just write a brief statement that explains how you'll bring "diversity" to the program through your interests and experiences. You can also briefly mention your cultural background. The key thing is to keep this concise and not to say things that you wouldn't feel comfortable saying to a professor in person over a conversation. The common trap is to "pour your guts out" or to pander by making tenuous claims.

If this is an optional statement, you can skip it (particularly in STEM fields), but most people use this as an opportunity to "humanize" themselves by telling non-work-related stories.