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/image.js

@@ -38,7 +38,7 @@ ol.Image = function(extent, resolution, pixelRatio, attributions, src,
    * @type {HTMLCanvasElement|Image|HTMLVideoElement}
    */
   this.image_ = new Image();
-  if (crossOrigin) {
+  if (crossOrigin !== null) {
     this.image_.crossOrigin = crossOrigin;
   }