This allows for a conditional `extra` parameter to not throw an error when using babel programatically (i.e. not via `.babelrc`). For example:

```javascript
extra: {
  'react-transform': env !== 'production' ? [{
    target: 'react-transform-webpack-hmr',
    imports: [ 'react' ],
    locals: [ 'module' ]
  }] : [ ] // This would otherwise balk.
}
```

Arguably one could simply check if there are no `extra` values for the plugin and simply not load it.