dropdown menu with more than one page having child pages is not rendered

I wanted to create a dropdown with two elements, which have its own children. It ends up in a warning ```Warning: DOMNode::appendChild(): Document Fragment is empty``` and the second is not displayed.
Example navigation config to reproduce:
```
return array(
    'containers' => array(
        'container' => array(
            array(
                'name'       => 'EntryOne',
                'options' => array(
                    'label' => 'one',
                    'route' => 'one'
                ),
                'pages' =>array(
                    array(
                        'name'       => 'First',
                        'options' => array(
                            'label' => 'first',
                            'route' => 'first'
                        )
                    ),
                    array(
                        'name'       => 'Second',
                        'options' => array(
                            'label' => 'second',
                            'route' => 'second'
                        )
                    ),
                )
            ),
            array(
                'name'       => 'EntryTwo',
                'options' => array(
                    'label' => 'two',
                    'route' => 'two'
                ),
                'pages' =>array(
                    array(
                        'name'       => 'Third',
                        'options' => array(
                            'label' => 'third',
                            'route' => 'third'
                        )
                    ),
                    array(
                        'name'       => 'Fourth',
                        'options' => array(
                            'label' => 'fourth',
                            'route' => 'fourth'
                        )
                    ),
                ),
            ),
            // ...
```
Add test and closes #80
Add test and closes #80
fixed intend
Update NavigationMenu.php
Update NavigationMenuTest.php
Update NavigationMenu.php
Update NavigationMenuTest.php
Merge pull request #83 from manuscle/master

change null to empty for the test of makeFile property
Merge pull request #80 from siad007/patch-1

dropdown menu with more than one page having child pages is not rendered