[BUGFIX] Ensure extractDottedPathToLastElement() always returns a string

Resolves: #80919
Releases: master, 8.7
Change-Id: I3fddc6e83a117d25ec7abeb7d82130275321c2c7
Reviewed-on: https://review.typo3.org/52519


Reviewed-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Tested-by: Anja Leichsenring <aleichsenring@ab-softlab.de>
Reviewed-by: Jan Helke <typo3@helke.de>
Tested-by: Jan Helke <typo3@helke.de>
Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>