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 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.
Merge pull request #2107 from tschaub/null-or-undef

Account for null or undefined geometry.