[Form] Fixed validation

src/Symfony/Component/Form/Form.php

@@ -799,7 +799,7 @@ class Form implements \IteratorAggregate, FormInterface
             }
 
             if (null === $groups) {
-                $groups = array(null);
+                $groups = array('Default');
             }
 
             $propertyPath = $context->getPropertyPath();

tests/Symfony/Tests/Component/Form/Type/FormTypeTest.php

@@ -495,8 +495,8 @@ class FormTest extends TestCase
         $form = $this->factory->create('form', 'author');
 
         $graphWalker->expects($this->once())
-        ->method('walkReference')
-        ->with($object, null, 'path.data', true);
+            ->method('walkReference')
+            ->with($object, 'Default', 'path.data', true);
 
         $form->setData($object);
         $form->validateData($context);