The default value does not allow an empty string

When using the `DefaultableStorage` an ampty value of the `defaultValue` option is not allowed

```javascript
import SuperchargedStorage, {
  DefaultableStorage,
} from 'supercharged-storage';

SuperchargedStorage.use(DefaultableStorage);

const storage = SuperchargedStorage.build({
  defaultValue: '',
});

// throws "Please provide a valid defaultValue option"
```