minor #27954 [Form] Fix PHPDoc for FormConfigBuilder $dataClass argument (emodric)

This PR was merged into the 2.8 branch.

Discussion
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[Form] Fix PHPDoc for FormConfigBuilder $dataClass argument

| Q             | A
| ------------- | ---
| Branch?       | 2.8
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | N/A
| License       | MIT
| Doc PR        | N/A

`$dataClass` constructor argument of `FormConfigBuilder` obviously allows `null` as its value and it even has a `?string` typehint in `master` branch, so this fixes the PHPDoc typehint to match.

Commits
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9854a26981 [Form] Fix PHPDoc for FormConfigBuilder $dataClass argument

src/Symfony/Component/Form/FormConfigBuilder.php

@@ -138,7 +138,7 @@ class FormConfigBuilder implements FormConfigBuilderInterface
     private $data;
 
     /**
-     * @var string
+     * @var string|null
      */
     private $dataClass;
 
@@ -181,7 +181,7 @@ class FormConfigBuilder implements FormConfigBuilderInterface
      * Creates an empty form configuration.
      *
      * @param string|int               $name       The form name
-     * @param string                   $dataClass  The class of the form's data
+     * @param string|null              $dataClass  The class of the form's data
      * @param EventDispatcherInterface $dispatcher The event dispatcher
      * @param array                    $options    The form options
      *