[Form] fixed Form::setParent(null)

src/Symfony/Component/Form/Form.php

@@ -259,7 +259,7 @@ class Form implements \IteratorAggregate, FormInterface
             throw new AlreadyBoundException('You cannot set the parent of a bound form');
         }
 
-        if ('' === $this->config->getName()) {
+        if (null !== $parent && '' === $this->config->getName()) {
             throw new FormException('A form with an empty name cannot have a parent form.');
         }
 

src/Symfony/Component/Form/FormInterface.php

@@ -21,7 +21,7 @@ interface FormInterface extends \ArrayAccess, \Traversable, \Countable
     /**
      * Sets the parent form.
      *
-     * @param  FormInterface $parent The parent form
+     * @param  FormInterface|null $parent The parent form or null if it's the root.
      *
      * @return FormInterface The form instance
      */
@@ -30,7 +30,7 @@ interface FormInterface extends \ArrayAccess, \Traversable, \Countable
     /**
      * Returns the parent form.
      *
-     * @return FormInterface The parent form
+     * @return FormInterface|null The parent form or null if there is none.
      */
     public function getParent();