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

lib/executor.js

@@ -31,6 +31,9 @@ function Executor () {
       };
 
       newArguments[newArguments.length - 1] = callback;
+    } else if (!lastArg && task.arguments.length) {
+        callback = function () { cb(); };
+        newArguments[newArguments.length - 1] = callback;
     } else {
       callback = function () { cb(); };
       newArguments.push(callback);