Fix \Knp\Menu\MenuItem @Label issue

[VincentLanglet]

Subject

I am targeting this branch, because it is a bug fix.

Closes #6031

Changelog

### Fixed
- Call \Knp\Menu\MenuItem::getLabel() method directly in twig template to avoid a possible side effect from \ArrayAccess.

Description:

\Knp\Menu\MenuItem in knplabs/knp-menu-bundle implements \ArrayAccess and defines its offsetGet method as follows:

    /**
     * Implements ArrayAccess
     */
    public function offsetGet($name)
    {
        return $this->getChild($name);
    }

    public function getChild($name)
    {
        return isset($this->children[$name]) ? $this->children[$name] : null;
    }

It has a property $label with its getter:

    /**
     * Label to output, name is used by default
     *
     * @var string|null
     */
    protected $label;

    public function getLabel()
    {
        return (null !== $this->label) ? $this->label : $this->name;
    }

Problem :

If a MenuItem object has a child with the key label, the twig expression menu.label will trigger the offsetGet method of the \ArrayAccess and return the child instead of the $label property of the MenuItem object.

This would result in Catchable Fatal Error: Object of class Knp\Menu\MenuItem could not be converted to string.

Solution:

Call the getter of the $label property directly:

menu.getLabel()

[jordisala1991]

Thank you @VincentLanglet

[franmomu]

IMHO it would be nice to add the functional tests added in the original PR https://github.com/sonata-project/SonataAdminBundle/pull/6031/files

[VincentLanglet]

IMHO it would be nice to add the functional tests added in the original PR https://github.com/sonata-project/SonataAdminBundle/pull/6031/files

I added one, without creating a whole class/admin called PR6301.
The configureTabMenu I added was failing without this PR. And doesn't fail now. Isn't that enough ?

[franmomu]

Well, the current test do not test twig calls, #6031 (review). In fact, when the test was added, it found another place where was wrong.