Fixes getModelForSpec to return an empty collection

```fetcher.getModelForSpec``` is supposed to return an empty model / collection that afterwards can be fetched. But the current implementation always returns a collection with a single empty model in it, because the models function parameter of the Collection constructor is set to ```{}```.

Usually this is not an issue because the collection is fetched right afterwards. But we are building a collection for search and endless scrolling and it uses ```collection.length``` as an offset. This issue results in the first search result never being fetched (because the ```collection.length```is ```1```at initialization).