[BUGFIX] OpCache XCache cannot be cleared if xcache.admin.enable_auth

We can only clear the opcache in XCache if xcache.admin.enable_auth is not
set, else you get a fatal error.

Resolves: #56554
Related: #55252
Releases: 6.2
Change-Id: Ia33afc4141852c58266f6c7dfedec82f4c35148d
Reviewed-on: https://review.typo3.org/28059
Reviewed-by: Christian Kuhn
Tested-by: Christian Kuhn
Reviewed-by: Ernesto Baschny
Tested-by: Ernesto Baschny