[BUGFIX] Avoid GU::trimExplode('', null) in BU::getProcessedValue()

GeneralUtility::trimExplode() needs a string since #101453
as second argument. TCA type=folder DB values can be null,
which can be triggered for instance by bootstrap_package
in BackendUtility::getProcessedValue().
Cast the value before feeding it to GU::trimExplode().

Resolves: #102172
Related: #101453
Releases: main, 12.4
Change-Id: I3a742d592efe28324d07b0c941185601cfc14891
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/81429


Reviewed-by: Simon Schaufelberger <simonschaufi+typo3@gmail.com>
Reviewed-by: Stefan Bürk <stefan@buerk.tech>
Tested-by: Oliver Klee <typo3-coding@oliverklee.de>
Tested-by: Simon Schaufelberger <simonschaufi+typo3@gmail.com>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Stefan Bürk <stefan@buerk.tech>
Reviewed-by: Oliver Klee <typo3-coding@oliverklee.de>