I think this will break the idempotence of import/export:
```
xml (no <prompt>) --> XBlock --> xml (<prompt></prompt>)
```
This could be confusing for Studio authors (why does the prompt reappear after I delete it?) and makes it harder to reason about the file format.

For this reason, I'd prefer to distinguish between "no prompt" and "an empty prompt".  That is, I'd like the XBlock prompt field to be set to `None` iff the XML definition is missing the `<prompt>` element.