minor #33281 Backported return type violation bugfixes (derrabus)

This PR was merged into the 3.4 branch.

Discussion
----------

Backported return type violation bugfixes

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #33228
| License       | MIT
| Doc PR        | N/A

Fixed some doc blocks and broken return values discovered while working on #33279.

Commits
-------

8877a013d7 Backported return type violation bugfixes.

src/Symfony/Component/Form/Tests/Extension/Validator/EventListener/ValidationListenerTest.php

@@ -26,6 +26,7 @@ use Symfony\Component\Form\FormFactoryBuilder;
 use Symfony\Component\Form\FormFactoryInterface;
 use Symfony\Component\Validator\ConstraintViolation;
 use Symfony\Component\Validator\ConstraintViolationInterface;
+use Symfony\Component\Validator\ConstraintViolationList;
 use Symfony\Component\Validator\Context\ExecutionContextInterface;
 use Symfony\Component\Validator\Validation;
 use Symfony\Component\Validator\Validator\ValidatorInterface;
@@ -166,7 +167,7 @@ class DummyValidator implements ValidatorInterface
 
     public function validate($value, $constraints = null, $groups = null)
     {
-        return [$this->violation];
+        return new ConstraintViolationList([$this->violation]);
     }
 
     public function validateProperty($object, $propertyName, $groups = null)

src/Symfony/Component/Form/Tests/Fixtures/FooType.php

@@ -17,5 +17,6 @@ class FooType extends AbstractType
 {
     public function getParent()
     {
+        return null;
     }
 }

src/Symfony/Component/Security/Core/Encoder/EncoderAwareInterface.php

@@ -22,7 +22,7 @@ interface EncoderAwareInterface
      * If the method returns null, the standard way to retrieve the encoder
      * will be used instead.
      *
-     * @return string
+     * @return string|null
      */
     public function getEncoderName();
 }

src/Symfony/Component/Security/Core/User/User.php

@@ -69,6 +69,7 @@ final class User implements AdvancedUserInterface
      */
     public function getSalt()
     {
+        return null;
     }
 
     /**