[TASK] Fix return type annotation of get_dirs method

As three different return types can be returned, they should all be
documented. This allows for better autocompletion and type safety through
static code analysis.

Resolves: #89407
Releases: master, 9.5, 8.7
Change-Id: I181bc9b79eeaa4add0d2d06cfd6b8b2fda330442
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/61984
Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Frank Nägler <frank.naegler@typo3.org>
Reviewed-by: Frank Nägler <frank.naegler@typo3.org>

typo3/sysext/core/Classes/Utility/GeneralUtility.php

@@ -2267,9 +2267,10 @@ public static function flushDirectory($directory, $keepOriginalDirectory = false
     /**
      * Returns an array with the names of folders in a specific path
      * Will return 'error' (string) if there were an error with reading directory content.
+     * Will return null if provided path is false.
      *
      * @param string $path Path to list directories from
-     * @return array Returns an array with the directory entries as values. If no path, the return value is nothing.
+     * @return array|string|null Returns an array with the directory entries as values. If no path is provided, the return value will be null.
      */
     public static function get_dirs($path)
     {