Allow '' for crossOrigin (as Anonymous alias)

This fixes a regression introduced by the goog.isDef removal.

From the HTML 5 spec: "The empty string is also a valid keyword, and
maps to the Anonymous state"

src/ol/style/iconstyle.js

@@ -392,7 +392,7 @@ ol.style.IconImage_ = function(image, src, size, crossOrigin, imageState) {
    */
   this.image_ = !image ? new Image() : image;
 
-  if (crossOrigin) {
+  if (crossOrigin !== null) {
     this.image_.crossOrigin = crossOrigin;
   }