[BUGFIX] Installer triggers PHP warnings with folder structure tests

Resolves: #49700
Releases: 6.2
Change-Id: I0e77811b733141a3e70a9dba0c203406bc0571a6
Reviewed-on: https://review.typo3.org/21930
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters