Make sure callback isn't executed synchronously

lib/canvas.js

@@ -261,7 +261,13 @@ Canvas.prototype.toDataURL = function(a1, a2, a3){
 
   if (this.width === 0 || this.height === 0) {
     // Per spec, if the bitmap has no pixels, return this string:
-    return fn ? fn(null, "data:,") : "data:,";
+    if (fn) {
+      setTimeout(function() {
+        fn(null, "data:,");
+      });
+      return;
+    }
+    return "data:,";
   }
 
   if ('image/png' === type) {