Small optimisation of _groupMembers
Small optimisation of _groupMembers

This will not change the result as users are check to be existing
 afterwards but avoids this check when we know it’s a group.

Signed-off-by: Côme Chilliet <65266482bbd411837c7b5a3a962855e6c075a719@nextcloud.com>

nextcloud/server,nextcloud/server,nextcloud/server,nextcloud/server