minor #18158 [Form] Add missing return type (hbgamra)

This PR was submitted for the 6.3 branch but it was merged into the 6.2 branch instead.

Discussion
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[Form] Add missing return type

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Commits
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179f26e Fix bug return type of closure

Author: javiereguiluz

form/use_empty_data.rst

@@ -94,7 +94,7 @@ The closure must accept a ``FormInterface`` instance as the first argument::
     public function configureOptions(OptionsResolver $resolver): void
     {
         $resolver->setDefaults([
-            'empty_data' => function (FormInterface $form) {
+            'empty_data' => function (FormInterface $form): Blog {
                 return new Blog($form->get('title')->getData());
             },
         ]);