[BUGFIX] Prevent Extbase warnings accessing ObjectStorages with strings

Feeding ObjectAccess->getPropertyPath() with an ObjectStorage raises
warnings, that might flood the errorLog quickly.

With the patch those warnings are prevented.
Additionaly the patch delivers a test scenario explaining the missing
ability to properly resolve the properny path, when it comes to
ObjectStorages.

Fixes: #37000
Releases: 6.1
Change-Id: I3acf5aef281894f06f1e35f45ac14ba692a90b3e
Reviewed-on: https://review.typo3.org/18527
Reviewed-by: Wouter Wolters
Reviewed-by: Pascal Dürsteler
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Tested-by: Wouter Wolters
Reviewed-by: Stefan Neufeind
Tested-by: Stefan Neufeind
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring