You're assuming all the questions are being presented to the user at once. Imagine a UI that presents questions to a user one-by-one. If the user bails out of the survey early, the rest of the survey answers can be null.
I think the null vs empty argument works better when there are 40 boxes on screen at once. But again I'm not sure what semantic difference you'd find between 'they left it empty' and 'they never clicked into it'.
Second, in Dynamo you could chose the absence of a key to represent "never got here" and a key with a null value to represent "they left this blank". You don't need an empty string.
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.