minor #10647 [Form] Improved test coverage of UrlType (webmozart)

This PR was submitted for the master branch but it was merged into the 2.3 branch instead (closes #10647).

Discussion
----------

[Form] Improved test coverage of UrlType

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

Commits
-------

f3e172f [Form] Improved test coverage of UrlType

src/Symfony/Component/Form/Tests/Extension/Core/Type/UrlTypeTest.php

@@ -47,6 +47,18 @@ public function testSubmitAddsNoDefaultProtocolIfEmpty()
         $this->assertSame('', $form->getViewData());
     }
 
+    public function testSubmitAddsNoDefaultProtocolIfNull()
+    {
+        $form = $this->factory->create('url', null, array(
+            'default_protocol' => 'http',
+        ));
+
+        $form->submit(null);
+
+        $this->assertNull($form->getData());
+        $this->assertSame('', $form->getViewData());
+    }
+
     public function testSubmitAddsNoDefaultProtocolIfSetToNull()
     {
         $form = $this->factory->create('url', null, array(