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.

src/ol/render/canvas/canvasimmediate.js

@@ -462,7 +462,7 @@ ol.render.canvas.Immediate.prototype.drawCircleGeometry =
  */
 ol.render.canvas.Immediate.prototype.drawFeature = function(feature, style) {
   var geometry = feature.getGeometry();
-  if (goog.isNull(geometry) ||
+  if (!goog.isDefAndNotNull(geometry) ||
       !ol.extent.intersects(this.extent_, geometry.getExtent())) {
     return;
   }