Remove undefined when using `mantra generate collection`

[adrianliaw]

The initial lib/collections/index.js looks like this:

export {
  undefined
};

So, after running mantra generate collection posts, it did add posts to the export object, but undefined was still there:

import posts from "./posts";

export {
  undefined
  posts
}

We need a way to remove that undefined when generating the first collection.

[sungwoncho]

undefined is there because of a babel issue see https://github.com/meteor/meteor/issues/6281. It will be removed when the issue is resolved. Therefore it is not a high priority. You can still do a PR.