We should not allow for empty fragments fields should be selected

[mikkoh]

I don't think we should allow users to create empty fragments for instance in this case:

const graph = new Graph('Shop');

assert.deepEqual(splitQuery(graph.toQuery()), splitQuery('fragment on Shop { }'));

We should probably throw an Error when the user calls graph.toQuery(). Basically be pre-emptive on helping the user before they get errors back from the API.

[mikkoh]

I've been thinking about this more and maybe the query should actually just return a blank string and log a warning that they're wanting to query Shop but are not selecting any fields.

I'm just thinking from a UI perspective. If a parent component sais something like:
"I am a Shop component therefore we'll need to query on Shop"
But then child UI components don't actually query any of the fields.

Thoughts @minasmart @jamesmacaulay?

[minasmart]

I think we should error out if they don't add selections to a field that has a kind of OBJECT. Any such query would fail, since all queries against an OBJECT kind require selections. I think I'd error at the point when addField is called and no selections are added.

[minasmart]

Also, my opinion is the sane regardless of wether this is a fragment or a field. We know the kind either way.