[TASK] Make filelist PHP8 compatible

Added fallback value for array undefined
keys for filelist as well as for impexp
because it is part of the current clipboard
fuctionality. Introduced new tests for
clipboard and file upload.

Resolves: #94509
Releases: master
Change-Id: Ibed7c53f49665c4502aaa05dbe78f468d354f3a1
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/69771


Tested-by: Oliver Bartsch <bo@cedev.de>
Tested-by: core-ci <typo3@b13.com>
Tested-by: Benni Mack <benni@typo3.org>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Benni Mack <benni@typo3.org>