Always write the GeoJSONFeature geometry property

src/ol/format/geojsonformat.js

@@ -541,6 +541,8 @@ ol.format.GeoJSON.prototype.writeFeatureObject = function(
   if (goog.isDefAndNotNull(geometry)) {
     object['geometry'] =
         ol.format.GeoJSON.writeGeometry_(geometry, opt_options);
+  } else {
+    object['geometry'] = null;
   }
   var properties = feature.getProperties();
   goog.object.remove(properties, feature.getGeometryName());

test/spec/ol/format/geojsonformat.test.js

@@ -530,6 +530,12 @@ describe('ol.format.GeoJSON', function() {
       expect(geojson.properties).to.eql(null);
     });
 
+    it('writes out a feature without geometry correctly', function() {
+      var feature = new ol.Feature();
+      var geojson = format.writeFeatureObject(feature);
+      expect(geojson.geometry).to.eql(null);
+    });
+
   });
 
   describe('#writeGeometry', function() {