zoomtolayer only when there are features

We have a very rare scenario where we use views_geojson maps with a bounding box strategy.

This makes the loadend event fire twice: once with features, once without.

The provided patch checks if the layers currently has features (in the zoomtolayer javascript) and only continues if there are features. Which does make total sense, there is nothing to zoom without features anyways.

And it fixes our issue.