[BUGFIX] Cookies are set and start output in 6.2

Override the user authentication via XClass and use
an emtpy body for setSessionCookie.

Fixes: #59956
Change-Id: I156d4e895ed5572a211075d6c85e915f34973b7c
Reviewed-on: https://review.typo3.org/31178
Reviewed-by: Oliver Klee
Tested-by: Oliver Klee