Add ol.geom.GeometryCollection#isEmpty

src/ol/geom/geometrycollection.js

@@ -173,6 +173,14 @@ ol.geom.GeometryCollection.prototype.getType = function() {
 };
 
 
+/**
+ * @return {boolean} Is empty.
+ */
+ol.geom.GeometryCollection.prototype.isEmpty = function() {
+  return goog.array.isEmpty(this.geometries_);
+};
+
+
 /**
  * @param {Array.<ol.geom.Geometry>} geometries Geometries.
  */