ad-connector: fix all users revoked when AD is down

Fix f037a08d didn't work for active directory since
UserProfileService.getUserProfile returns an empty array instead of null
for Active Directory. In this case we must also check for an empty array
in order to identify that the AD server is down.

Ticket: https://redmine.savoirfairelinux.com/issues/7656
Change-Id: Icda219ec5f85b5b40e7a0c56957b2eb1dcd4f866