Evaluate helpers without arguments as getters

EJS isn't printing out a helper correctly if I do the following:

```js
environment.registerHelper('appName', function() {
  return "coolAppName";
});
```

```ejs
<%= appName %>
```

```js
// Output
function () {
  return helper.func.apply(self, arguments);
}
```

With my change, it checks the length of the helper and if there are
no arguments it just calls the function and adds it as a getter:

```js
// New Output
coolAppName
```