bug #33353 Return null as Expire header if it was set to null (danrot)

This PR was squashed before being merged into the 3.4 branch (closes #33353).

Discussion
----------

Return null as Expire header if it was set to null

| Q             | A
| ------------- | ---
| Branch?       | 3.4 <!-- see below -->
| Bug fix?      | yes
| New feature?  | no <!-- please update src/**/CHANGELOG.md files -->
| BC breaks?    | no     <!-- see https://symfony.com/bc -->
| Deprecations? | no <!-- please update UPGRADE-*.md and src/**/CHANGELOG.md files -->
| Tests pass?   | yes    <!-- please add some, will be required by reviewers -->
| Fixed tickets |    <!-- #-prefixed issue number(s), if any -->
| License       | MIT
| Doc PR        | symfony/symfony-docs#... <!-- required for new features -->

This PR fixes a regression introduces in #33332. If you set the `Expires` header to null when creating a `Response`, the `getExpires` function returned a date instead of null.

```php
$response = new Response(null, 200, ['Expires' => null]);
$response->getExpires(); // Returns a date currently, but should return null
```

See also [the comment](https://github.com/symfony/symfony/pull/33332#discussion_r317934607) in the PR introducing this regression.

Commits
-------

5e3c7ea452 Return null as Expire header if it was set to null

src/Symfony/Component/HttpFoundation/HeaderBag.php

@@ -121,7 +121,15 @@ class HeaderBag implements \IteratorAggregate, \Countable
         }
 
         if ($first) {
-            return \count($headers[$key]) ? (string) $headers[$key][0] : $default;
+            if (!$headers[$key]) {
+                return $default;
+            }
+
+            if (null === $headers[$key][0]) {
+                return null;
+            }
+
+            return (string) $headers[$key][0];
         }
 
         return $headers[$key];

src/Symfony/Component/HttpFoundation/Tests/HeaderBagTest.php

@@ -48,6 +48,13 @@ class HeaderBagTest extends TestCase
         $this->assertInstanceOf('DateTime', $headerDate);
     }
 
+    public function testGetDateNull()
+    {
+        $bag = new HeaderBag(['foo' => null]);
+        $headerDate = $bag->getDate('foo');
+        $this->assertNull($headerDate);
+    }
+
     public function testGetDateException()
     {
         $this->expectException('RuntimeException');
@@ -96,6 +103,9 @@ class HeaderBagTest extends TestCase
         $bag->set('foo', 'bor', false);
         $this->assertEquals('bar', $bag->get('foo'), '->get return first value');
         $this->assertEquals(['bar', 'bor'], $bag->get('foo', 'nope', false), '->get return all values as array');
+
+        $bag->set('baz', null);
+        $this->assertNull($bag->get('baz', 'nope'), '->get return null although different default value is given');
     }
 
     public function testSetAssociativeArray()

src/Symfony/Component/HttpFoundation/Tests/ResponseTest.php

@@ -369,6 +369,12 @@ class ResponseTest extends ResponseTestCase
         $this->assertNull($response->headers->get('Expires'), '->expire() removes the Expires header when the response is fresh');
     }
 
+    public function testNullExpireHeader()
+    {
+        $response = new Response(null, 200, ['Expires' => null]);
+        $this->assertNull($response->getExpires());
+    }
+
     public function testGetTtl()
     {
         $response = new Response();