[BUGFIX] Fix uninitialized string offset when renaming folder

On PHP 8.0, renaming a folder in the backend might fail
with a "PHP Warning: Uninitialized string offset 0". This
is fixed by utilizing the substr() function instead.

Resolves: #95273
Releases: master
Change-Id: Ib642a25492c933b6fae857206c4535b7a861cc3c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/71123


Tested-by: core-ci <typo3@b13.com>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Georg Ringer <georg.ringer@gmail.com>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Georg Ringer <georg.ringer@gmail.com>

typo3/sysext/core/Classes/Resource/StorageRepository.php

@@ -397,7 +397,7 @@ class StorageRepository implements LoggerAwareInterface
         }
         // normalize path information (`//`, `../`)
         $localPath = PathUtility::getCanonicalPath($localPath);
-        if ($localPath[0] !== '/') {
+        if (!str_starts_with($localPath, '/')) {
             $localPath = '/' . $localPath;
         }
         $bestMatchStorageUid = 0;