Advice for turning down an Exit Interview?

I already know they're optional and I have nothing good to say about the company, so I won't be doing one.

My supervisor is supposed to get back to me soon about if HR wants to do one at all, because she wasn't sure. (She's on the long laundry list of reasons why I am leaving. She's not a great manager.)

The next time we speak, I know there's a very good chance she will ask "Why not?" In lieu of "None of your damn business," what are your guys' suggestions for how to politely turn down an Exit Interview in a to-the-point and acceptable manner?

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