Fix Menu bug when using `template` with `encodeLabel` => false

```php
Menu::widget([
        'encodeLabels' => false,
        'items' => [
            ['template' => '<div class="sidebar-toggler"></div>'],
            ['label' => 'Articles', 'url' => ['/article/index']],
        ],
    ])
```
Lead to exception: 
```
PHP Notice – yii\base\ErrorException
Undefined index: label
```