minor #31963 [Form] test case is not legacy (nicolas-grekas)

This PR was merged into the 3.4 branch.

Discussion
----------

[Form] test case is not legacy

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

At least it's green.
/cc @xabbuh @HeahDude

Commits
-------

9ad324ba29 [Form] test case is not legacy

src/Symfony/Component/Form/Tests/Extension/Core/Type/ChoiceTypeTest.php

@@ -725,10 +725,7 @@ class ChoiceTypeTest extends BaseTypeTest
         $this->assertSame(['test'], $form->getData());
     }
 
-    /**
-     * @group legacy
-     */
-    public function testLegacyNullChoices()
+    public function testNullChoices()
     {
         $form = $this->factory->create(static::TESTED_TYPE, null, [
             'multiple' => false,