[BUGFIX] Handle getFileContents in LocalDriver better

The method getFileContents has return value string. But since
there is no guarantee that the file exists before calling
file_get_contents, a call to is_readable has been added.
This handles both the PHP Warning and ensures type correctness.

Resolves: #99926
Releases: main, 11.5
Change-Id: I9786a6737d7e3b0c3d04272364d868fd4101307c
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/77989
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: core-ci <typo3@b13.com>

typo3/sysext/core/Classes/Resource/Driver/LocalDriver.php

@@ -1351,12 +1351,12 @@ public function createFile($fileName, $parentFolderIdentifier)
      * processing resources and money) for large files.
      *
      * @param string $fileIdentifier
-     * @return string The file contents
+     * @return string The file contents if file exists and else empty string
      */
     public function getFileContents($fileIdentifier)
     {
         $filePath = $this->getAbsolutePath($fileIdentifier);
-        return file_get_contents($filePath);
+        return is_readable($filePath) ? (string)file_get_contents($filePath) : '';
     }
 
     /**