minor #30535 Fix return type of Request::getRequestFormat (teohhanhui)

This PR was merged into the 3.4 branch.

Discussion
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Fix return type of Request::getRequestFormat

| Q             | A
| ------------- | ---
| Branch?       | 3.4
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | N/A
| License       | MIT
| Doc PR        | N/A

Helps static analysis tools get the correct type.

Commits
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bd92349 Fix return type of Request::getRequestFormat

src/Symfony/Component/HttpFoundation/Request.php

@@ -1462,7 +1462,7 @@ public function setFormat($format, $mimeTypes)
      *
      * @param string|null $default The default format
      *
-     * @return string The request format
+     * @return string|null The request format
      */
     public function getRequestFormat($default = 'html')
     {