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/imagetile.js

@@ -37,7 +37,7 @@ ol.ImageTile = function(tileCoord, state, src, crossOrigin, tileLoadFunction) {
    * @type {Image}
    */
   this.image_ = new Image();
-  if (crossOrigin) {
+  if (crossOrigin !== null) {
     this.image_.crossOrigin = crossOrigin;
   }