bug #42380 [HttpFoundation] Don't pass null to strpos() (derrabus)

This PR was merged into the 5.3 branch.

Discussion
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[HttpFoundation] Don't pass null to strpos()

| Q             | A
| ------------- | ---
| Branch?       | 5.3
| Bug fix?      | yes
| New feature?  | no
| Deprecations? | no
| Tickets       | #41552
| License       | MIT
| Doc PR        | N/A

Passing `null` to `strpos()` triggers a deprecation warning in PHP 8.1. Let's not do that.

Commits
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bf9dca3f17 Don't pass null to strpos()

Author: fabpot

Request.php

@@ -1333,7 +1333,7 @@ public static function getMimeTypes(string $format)
     public function getFormat(?string $mimeType)
     {
         $canonicalMimeType = null;
-        if (false !== $pos = strpos($mimeType, ';')) {
+        if ($mimeType && false !== $pos = strpos($mimeType, ';')) {
             $canonicalMimeType = trim(substr($mimeType, 0, $pos));
         }