Merge pull request #4360 from fredj/getExtent_undef

Update getExtent return value jsdoc tag

src/ol/source/vectorsource.js

@@ -659,7 +659,7 @@ ol.source.Vector.prototype.getClosestFeatureToCoordinate =
  *
  * This method is not available when the source is configured with
  * `useSpatialIndex` set to `false`.
- * @return {ol.Extent} Extent.
+ * @return {!ol.Extent} Extent.
  * @api stable
  */
 ol.source.Vector.prototype.getExtent = function() {

src/ol/structs/rbush.js

@@ -253,7 +253,7 @@ ol.structs.RBush.prototype.clear = function() {
 
 /**
  * @param {ol.Extent=} opt_extent Extent.
- * @return {ol.Extent} Extent.
+ * @return {!ol.Extent} Extent.
  */
 ol.structs.RBush.prototype.getExtent = function(opt_extent) {
   // FIXME add getExtent() to rbush