minor symfony#61676 use the empty string instead of null as an array offset (xabbuh)

This PR was merged into the 7.3 branch.

Discussion
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use the empty string instead of null as an array offset

| Q             | A
| ------------- | ---
| Branch?       | 7.3
| Bug fix?      | no
| New feature?  | no
| Deprecations? | no
| Issues        |
| License       | MIT

Commits
-------

5b245bf use the empty string instead of null as an array offset

Author: fabpot

src/Symfony/Component/Mime/FileinfoMimeTypeGuesser.php

@@ -53,7 +53,7 @@ public function guessMimeType(string $path): ?string
         }
 
         try {
-            $finfo = self::$finfoCache[$this->magicFile] ??= new \finfo(\FILEINFO_MIME_TYPE, $this->magicFile);
+            $finfo = self::$finfoCache[$this->magicFile ?? ''] ??= new \finfo(\FILEINFO_MIME_TYPE, $this->magicFile);
         } catch (\Exception $e) {
             throw new RuntimeException($e->getMessage());
         }