[BUGFIX] Fix undefined array key warnings

When comparing user groups, a fake user record
is created to fetch the group data for. This led to
a couple of undefined array key warnings, which
are now fixed.

Resolves: #96038
Releases: master, 11.5
Change-Id: Ia1ed0dcf4483bf5b8bef26992c8943284c32de29
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/72238


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