sudo: do not store usn if no rules are found
When ldap doesn't contain any sudorule during the initial full refresh,
usn is set to 1 instead of remaining unset and we are trying to
search modifyTimestamp>=1 during smart refresh which doesn't return any result
on openldap servers.
Resolves:
https://fedorahosted.org/sssd/ticket/3257
Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
(cherry picked from commit 46703740e83a66909974a5ee8d47df6a6e5076e7)
(cherry picked from commit 76e97affaa05ce45709efd59d120595c5992aa21)