[BUGFIX] Perform is_dir without trailing slash

Workaround a PHP issue which will produce an open_basedir
restriction warning if `is_dir()` is used on a regular file
when the trailing slash is provided.

Resolves: #103943
Releases: main, 12.4, 11.5
Change-Id: Iaa83658113ff8abb0ab8051b519f76135effd627
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/84461


Tested-by: Benjamin Franzke <ben@bnf.dev>
Tested-by: core-ci <typo3@b13.com>
Reviewed-by: Benjamin Franzke <ben@bnf.dev>