[BUGFIX] Only load folder contents if folder is initialised

FolderBasedFileCollection::loadContents() has to check if 
the folder is initialised prior to loading the contents to
prevent fatal errors.

Fixes: #50802
Releases: 6.2, 6.1, 6.0
Change-Id: If3ce06cff13595da49abbb6aa99c891969aea692
Reviewed-on: https://review.typo3.org/24781
Reviewed-by: Markus Klein
Tested-by: Markus Klein