[Bug]: Followup CSF factories: codemod generates broken code in projects where the preview file doesn't have exports

[ndelangen]

Describe the bug

If a project has a preview.js file like so:

import './preview.scss'

Then the changes for that preview file will not provide a default export, which is required by any story file like:

import preview from '../../../../../.storybook/preview'

And therefore cause the stories to be broken

Same issue happens when the preview file is empty: