When last argument is falsy, Override by default callback instead of appending it

lib/executor.js

@@ -30,6 +30,9 @@ function Executor () {
         lastArg.apply(null, arguments);
       };
 
+      newArguments[newArguments.length - 1] = callback;
+    } else if (!lastArg && task.arguments.length) {
+        callback = function () { cb(); };
         newArguments[newArguments.length - 1] = callback;
     } else {
       callback = function () { cb(); };