[BUGFIX] Fall back to default folder when user-specific upload folder does not exist

When the userTSconfig options.defaultUserFolder = 1:/my-folder/ does not exist,
Backend users now fall back to the first selected user folder.

A test is added to show that the fallback to the default storage works.

Resolves: #86815
Releases: master, 10.4
Change-Id: Ia88a9020adb4ba522e10e3a6059189500d9a407f
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/64552


Tested-by: TYPO3com <noreply@typo3.com>
Tested-by: Markus Klein <markus.klein@typo3.org>
Tested-by: Daniel Goerz <daniel.goerz@posteo.de>
Reviewed-by: Andreas Fernandez <a.fernandez@scripting-base.de>
Reviewed-by: Markus Klein <markus.klein@typo3.org>
Reviewed-by: Oliver Hader <oliver.hader@typo3.org>
Reviewed-by: Daniel Goerz <daniel.goerz@posteo.de>