[BUGFIX] Correct an always-true if statement in GeneralUtility::mkdir_deep

Resolves: #82697
Releases: master, 8.7, 7.6
Change-Id: I64bf51d5efd0394f4d3269965820281cbaf2de90
Reviewed-on: https://review.typo3.org/54317


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>