[BUGFIX] Ensure processing folder within another storage is created

This patch checks if the processing folder within another storage
exists and ensures it is created if not.

Resolves: #70481
Releases: master, 6.2
Change-Id: Ib6747b5e37a46e6f761912103d1ac5f490bb2a1d
Reviewed-on: https://review.typo3.org/43903


Reviewed-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Tested-by: Stefan Neufeind <typo3.neufeind@speedpartner.de>
Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Michael Oehlhof <typo3@oehlhof.de>
Tested-by: Michael Oehlhof <typo3@oehlhof.de>
Reviewed-by: Benni Mack <benni@typo3.org>
Tested-by: Benni Mack <benni@typo3.org>