minor #28230 [Form] fix data mapper return type in docblock (xabbuh)

This PR was merged into the 2.8 branch.

Discussion
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[Form] fix data mapper return type in docblock

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

Commits
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5bdc755 fix data mapper return type in docblock

src/Symfony/Component/Form/FormConfigInterface.php

@@ -99,7 +99,7 @@ public function getModelTransformers();
     /**
      * Returns the data mapper of the form.
      *
-     * @return DataMapperInterface The data mapper
+     * @return DataMapperInterface|null The data mapper
      */
     public function getDataMapper();