Generating valid PHP code within visit method

Within the `visit` method, if the `$element` is an **Operator** and `$element->isFunction() === true` and this function is called *without any arguments* then an invalid PHP code is generated.

e.g.  
```
$model->func('function_name',)
```

When a method has no arguments, the comma that separates the method name from the list of the arguments needs to be removed .
hi,

I've removed the `{` and `}`

Thanks