Return no canvas data string synchronously for backwards compatibility

lib/canvas.js

@@ -261,13 +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:
+    var str = "data:,";
     if (fn) {
       setTimeout(function() {
-        fn(null, "data:,");
+        fn(null, str);
       });
-      return;
     }
-    return "data:,";
+    return str;
   }
 
   if ('image/png' === type) {