merged branch Flask/ticket_8548 (PR #8575)

This PR was squashed before being merged into the 2.3 branch (closes #8575).

Discussion
----------

[Form] fixes empty file-inputs get treated as extra field

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | #8548, #8566
| License       | MIT

Commits
-------

e5fba3c [Form] fixes empty file-inputs get treated as extra field

src/Symfony/Component/Form/Form.php

@@ -546,7 +546,7 @@ class Form implements \IteratorAggregate, FormInterface
 
             foreach ($this->children as $name => $child) {
                 $fieldValue = null;
-                if (isset($submittedData[$name])) {
+                if (array_key_exists($name, $submittedData)) {
                     $fieldValue = $submittedData[$name];
                     unset($submittedData[$name]);
                 }

src/Symfony/Component/Form/Tests/CompoundFormTest.php

@@ -13,6 +13,7 @@ namespace Symfony\Component\Form\Tests;
 
 use Symfony\Component\Form\Extension\HttpFoundation\HttpFoundationRequestHandler;
 use Symfony\Component\Form\FormError;
+use Symfony\Component\Form\Forms;
 use Symfony\Component\HttpFoundation\Request;
 use Symfony\Component\HttpFoundation\File\UploadedFile;
 use Symfony\Component\Form\Tests\Fixtures\FixedDataTransformer;
@@ -73,6 +74,19 @@ class CompoundFormTest extends AbstractFormTest
         $this->form->submit(array(), false);
     }
 
+    public function testSubmitDoesNotAddExtraFieldForNullValues()
+    {
+        $factory = Forms::createFormFactoryBuilder()
+            ->getFormFactory();
+
+        $child = $factory->create('file', null, array('auto_initialize' => false));
+
+        $this->form->add($child);
+        $this->form->submit(array('file' => null));
+
+        $this->assertCount(0, $this->form->getExtraData());
+    }
+
     public function testClearMissingFlagIsForwarded()
     {
         $child = $this->getMockForm('firstName');