[BUGFIX] Incorrect check for empty folder

The condition in ResourceStorage::deleteFolder is wrong.
Negate the isFolderEmpty() clause.

Resolves: #50125
Releases: 6.2, 6.1, 6.0
Change-Id: Ia60f3e97ff8d1e0cc2671cec1b31cfc995c235b1
Reviewed-on: https://review.typo3.org/22889
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters