bug #17430 [Serializer] Ensure that groups are strings (dunglas)

This PR was squashed before being merged into the 2.7 branch (closes #17430).

Discussion
----------

[Serializer] Ensure that groups are strings

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

Commits
-------

0a3b877 [Serializer] Ensure that groups are strings

Author: fabpot

Mapping/Loader/YamlFileLoader.php

@@ -74,7 +74,15 @@ public function loadClassMetadata(ClassMetadataInterface $classMetadata)
                     }
 
                     if (isset($data['groups'])) {
+                        if (!is_array($data['groups'])) {
+                            throw new MappingException('The "groups" key must be an array of strings in "%s" for the attribute "%s" of the class "%s".', $this->file, $attribute, $classMetadata->getName());
+                        }
+
                         foreach ($data['groups'] as $group) {
+                            if (!is_string($group)) {
+                                throw new MappingException('Group names must be strings in "%s" for the attribute "%s" of the class "%s".', $this->file, $attribute, $classMetadata->getName());
+                            }
+
                             $attributeMetadata->addGroup($group);
                         }
                     }