[BUGFIX] No double slash during folder canonicalization

The canonicalizeAndCheckFolderIdentifier does now check if the given
folder path is just a simple slash (/) and return it before really
canonicalizing the identifier.

This prevents the method returning a double slash (//) which will result
in an error in the isWithin() checkin the local driver.

Resolves: #55420
Releases: 6.2
Change-Id: Ibe189180a459377d042731931066a410e1e5ac51
Reviewed-on: https://review.typo3.org/27122
Reviewed-by: Frans Saris
Tested-by: Frans Saris