minor #12529 [2.3] Remove possible call_user_func() (nicolas-grekas)

This PR was merged into the 2.3 branch.

Discussion
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[2.3] Remove possible call_user_func()

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

Merging this in 2.3 enhances performance a bit, but more importantly will ease future merges into 3.0.

Commits
-------

fad7aba [2.3] Remove possible call_user_func()

Author: fabpot

Tests/HttpCache/TestHttpKernel.php

@@ -74,8 +74,8 @@ public function callController(Request $request)
 
         $response = new Response($this->body, $this->status, $this->headers);
 
-        if (null !== $this->customizer) {
-            call_user_func($this->customizer, $request, $response);
+        if (null !== $customizer = $this->customizer) {
+            $customizer($request, $response);
         }
 
         return $response;