Better class attribute rendering

When setting the `firstClass` and/or `lastClass` options to `null` or an empty string, like this:

```
{{ knp_menu_render('AcmeDemoBundle:Builder:mainMenu', { 'firstClass': null, 'lastClass': null }) }}
```

A menu with the following `class` attributes is rendered:

```

• ...


• ...

Even though this is completely valid markup, it would be nice if the trailing space after the class `current` would be gone, as well as the empty `class` attributes:

```

• ...


• ...

Start at the renderer behind the knp_menu_render call and trace handling of the firstClass and lastClass options. Done means null or empty options produce no trailing class-space and omit empty class attributes, matching the rendered examples.

Written by the indexing model from the issue text.