Return the no pixel string async if a callback was passed in

lib/canvas.js

@@ -231,11 +231,6 @@ Canvas.prototype.toDataURL = function(a1, a2, a3){
   // ['image/jpeg', qual, fn] -> ['image/jpeg', {quality: qual}, fn]
   // ['image/jpeg', undefined, fn] -> ['image/jpeg', null, fn]
 
-  if (this.width === 0 || this.height === 0) {
-    // Per spec, if the bitmap has no pixels, return this string:
-    return "data:,";
-  }
-
   var type = 'image/png';
   var opts = {};
   var fn;
@@ -264,6 +259,11 @@ 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 ('image/png' === type) {
     if (fn) {
       this.toBuffer(function(err, buf){