Don't throw exceptions when encoding empty geometries to GeoJSON

This PR fixes a bug that was causing the GeoJSON encoder to throw exceptions when asked to encode empty geometries.  GeoJSON specifies that geometries with no coordinates are invalid so I treated such geometries as equivalent to null.  It is no longer the case that encoding followed by decoding will give back an equal value but I believe it is worth it to avoid the unhandled exception.
That makes a lot of sense.  I'll make that change.