[BUGFIX] Fix fatal when processing folder does not exist

When the processing folder does not exist,
it is created on the fly in ResourceStorage.

However the returned object is at a later point
as string, leading to a fatal error in the end.

Solution is to just store the retuned object directly.

Resolves: #55471
Releases: 6.2
Change-Id: I97b4b5c7992db942812ae033fb963908a4abffde
Reviewed-on: https://review.typo3.org/27137
Reviewed-by: Steffen Ritter
Reviewed-by: Marcin Sągol
Reviewed-by: Anja Leichsenring
Tested-by: Anja Leichsenring
Reviewed-by: Helmut Hummel
Tested-by: Helmut Hummel