[BUGFIX] Fix phpstan finding in Clipboard

The parent folder could theoretically be an InaccessibleFolder
according to the return type annotation, but only Folder has the method
"getCombinedIdentifier".

Resolves: #100920
Releases: main, 12.4, 11.5
Change-Id: Ida8ade208279535c06af586b86413787339c5991
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/79158


Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Tested-by: Nikita Hovratov <nikita.h@live.de>
Tested-by: core-ci <typo3@b13.com>