bug symfony#21155 [Validator] Check cascasdedGroups for being countable (scaytrase)

This PR was squashed before being merged into the 2.7 branch (closes symfony#21155).

Discussion
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[Validator] Check cascasdedGroups for being countable

Prevents notice for PHP 7.2

| Q             | A
| ------------- | ---
| Branch?       | 2.7
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | -
| License       | MIT
| Doc PR        |

Just repeated for this place symfony#20859
Waiting travis to fill in the `Tests pass` field

`cascasdedGroups` can be null at this point (according to failures and phpdoc)

Commits
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8fa45a1 [Validator] Check cascasdedGroups for being countable

Author: fabpot

src/Symfony/Component/Validator/Validator/RecursiveContextualValidator.php

@@ -746,9 +746,7 @@ private function validateGenericNode($value, $object, $cacheKey, MetadataInterfa
         // The $cascadedGroups property is set, if the "Default" group is
         // overridden by a group sequence
         // See validateClassNode()
-        $cascadedGroups = count($cascadedGroups) > 0
-            ? $cascadedGroups
-            : $groups;
+        $cascadedGroups = null !== $cascadedGroups && count($cascadedGroups) > 0 ? $cascadedGroups : $groups;
 
         if (is_array($value)) {
             // Arrays are always traversed, independent of the specified