Don't fail when no featues are found in ol.format.TopoJSON

With this PR, `ol.format.TopoJSON` no longer raises an exception if no features can be found. This is so that `ol.interaction.DragAndDrop`, which tries multiple formats in succession, can function correctly. With the `goog.asserts.fail()` in place, `ol.format.TopoJSON` raises an exception if it's given data in a different format (like GeoJSON).