[BUGFIX] Prevent workspace access checks for dummy user

The backend usergroup compare view requires to fetch the
group data of the groups to compare. Since this operation
depends on a user array, UserInformationService creates
such dummy user.

Since #93233 the dummy user array also contains a workspace
id `-99`.

To successfully overcome workspace access-checks, set
the admin-flag for this fake-user. Use workspace uid 0,
to avoid any database operations on workspace access
checks. Workspace 0 additionally prevents the log-message,
which led to the exception message described at #93894.

Resolves: #93894
Related: #93233
Releases: master, 11.5
Change-Id: Ieb5da95082cb6dfd5c030fbd04d735863d587510
Reviewed-on: https://review.typo3.org/c/Packages/TYPO3.CMS/+/70056
Tested-by: core-ci <typo3@b13.com>
Tested-by: Jochen <rothjochen@gmail.com>
Tested-by: Nikita Hovratov <nikita.h@live.de>
Tested-by: Oliver Bartsch <bo@cedev.de>
Reviewed-by: Jochen <rothjochen@gmail.com>
Reviewed-by: Nikita Hovratov <nikita.h@live.de>
Reviewed-by: Oliver Bartsch <bo@cedev.de>
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