Both of those are data unknown, and both would normally be null. You don't distinguish between known unknowns and unknown unknowns at the database level.
When you say "The user submitted an empty form field as their answer", you do have data. It's weird to use null for that.
Since you can leave a key absent in DynamoDB, it basically has two ways to say "data unknown" and no way to say "knowingly left blank".
You can repurpose null as you suggested, and it will work fine in isolation, but it will massively violate the principle of least surprise and could lead to painful bugs.
