avoid NullPointerException, if a GeoJON object is null

Such a NullPointerException might occour, if GeoJSON geometry contains a real null value. 

Example (not a fully valid one, see geometry):
{
  "type": "FeatureCollection",
  "features": [
    {
      "bbox": null,
      "geometry": null,
      "type": "Feature",
      "id": "someID",
      "properties": 
      {
        "caption": "A feature without geometry"
      }
    }
  ]
}