minor #51032 [HttpFoundation][HttpKernel] Fix deprecations when Content-Type is null (HypeMC)

This PR was merged into the 5.4 branch.

Discussion
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[HttpFoundation][HttpKernel] Fix deprecations when `Content-Type` is `null`

| Q             | A
| ------------- | ---
| Branch?       | 5.4
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       | -
| License       | MIT
| Doc PR        | -

Followup to #51000. Two more places I discovered were triggering deprecations when `Content-Type` is `null`.

Commits
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678d7be1b3 [HttpFoundation][HttpKernel] Fix deprecations when `Content-Type` is `null`

Author: nicolas-grekas

Response.php

@@ -298,7 +298,7 @@ public function prepare(Request $request)
             $charset = $this->charset ?: 'UTF-8';
             if (!$headers->has('Content-Type')) {
                 $headers->set('Content-Type', 'text/html; charset='.$charset);
-            } elseif (0 === stripos($headers->get('Content-Type'), 'text/') && false === stripos($headers->get('Content-Type'), 'charset')) {
+            } elseif (0 === stripos($headers->get('Content-Type') ?? '', 'text/') && false === stripos($headers->get('Content-Type') ?? '', 'charset')) {
                 // add the charset
                 $headers->set('Content-Type', $headers->get('Content-Type').'; charset='.$charset);
             }

Tests/ResponseTest.php

@@ -511,6 +511,16 @@ public function testContentTypeCharset()
         $this->assertEquals('text/css; charset=UTF-8', $response->headers->get('Content-Type'));
     }
 
+    public function testContentTypeIsNull()
+    {
+        $response = new Response('foo');
+        $response->headers->set('Content-Type', null);
+
+        $response->prepare(new Request());
+
+        $this->expectNotToPerformAssertions();
+    }
+
     public function testPrepareDoesNothingIfContentTypeIsSet()
     {
         $response = new Response('foo');