bug #15662 [Form][2.7][ Choice type] fixed groups with empty array. (aitboudad)

This PR was merged into the 2.7 branch.

Discussion
----------

[Form][2.7][ Choice type] fixed groups with empty array.

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #15634
| License       | MIT
| Doc PR        |  ~

- [x] test

Commits
-------

835aa78 [Form][2.7][ Choice type] fixed groups with empty array.

src/Symfony/Component/Form/ChoiceList/Factory/DefaultChoiceListFactory.php

@@ -172,6 +172,10 @@ class DefaultChoiceListFactory implements ChoiceListFactoryInterface
     private static function addChoiceViewsGroupedBy($groupBy, $label, $choices, $keys, &$index, $attr, $isPreferred, &$preferredViews, &$otherViews)
     {
         foreach ($groupBy as $key => $value) {
+            if (null === $value) {
+                continue;
+            }
+
             // Add the contents of groups to new ChoiceGroupView instances
             if (is_array($value)) {
                 $preferredViewsForGroup = array();

src/Symfony/Component/Form/Tests/ChoiceList/Factory/DefaultChoiceListFactoryTest.php

@@ -536,6 +536,7 @@ class DefaultChoiceListFactoryTest extends \PHPUnit_Framework_TestCase
         $list = new ArrayChoiceList(array(
             'Group 1' => array('A' => $this->obj1, 'B' => $this->obj2),
             'Group 2' => array('C' => $this->obj3, 'D' => $this->obj4),
+            'Group empty' => array(),
         ));
 
         $view = $this->factory->createView(