Property value expected type of string but got null

- `babel-plugin-codegen` version: ^3.0.0
- `node` version: 10.12.0
- `npm` (or `yarn`) version: 6.4.1

Relevant code or config

```javascript
// @codegen

const hello = 'hello';

module.exports = `
export default {};
`;
```

I was expecting the simple code above to work but it just gives the error mentioned in the title.


However, if we remove the line `const hello = 'hello'`, everything works as expected and `export default {};` it's correctly generated

Reproduce the error with the supplied babel-plugin-codegen example, then compare it with the version that removes the `const hello = 'hello'` declaration. Trace how the plugin processes the declaration and identify why it produces a null value instead of accepting the example; done means the original snippet generates `export default {};` without the type error.

Written by the indexing model from the issue text.