Account for null or undefined geometry

The change in #2098 made it so a feature's geometry could be `undefined`.  This is consistent with the return type for the [`getGeometry`](https://github.com/openlayers/ol3/blob/0e7f86ea17e3698a4e560fa5f06f509cb7e3427c/src/ol/feature.js#L81-L85) method.  Where calling code needs to ensure that it has a geometry instance, it can use `instanceof`, `goog.isDefAndNotNull()`, or test for a truthy value.

One alternative would be to have `getGeometry` return `{ol.geom.Geometry}` (allowing `null` but not `undefined`).  However, this would mean that `feature.get(feature.getGeometryName()) === feature.getGeometry()` was not always true.