minor #27983 [DX] Fix PHPDoc return type (Jean85)

This PR was merged into the 2.8 branch.

Discussion
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[DX] Fix PHPDoc return type

GetResponseEvent::getResponse() may return null too

| Q             | A
| ------------- | ---
| Branch?       | 2.8
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | none
| License       | MIT

While doing static analysis on my code, I got a false negative for this method, since it may return null, but the PHPDoc says otherwise.

Commits
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8e49598788 Fix PHPDoc return type

src/Symfony/Component/HttpKernel/Event/GetResponseEvent.php

@@ -29,7 +29,7 @@ class GetResponseEvent extends KernelEvent
     /**
      * Returns the response object.
      *
-     * @return Response
+     * @return Response|null
      */
     public function getResponse()
     {