LDAP Sync Error

Looks like we're getting into an error state on a single LDAP Group.

Example Logs from the issue:

2018-01-15_17:01:52.91443 2018-01-15T17:01:52.758Z 36325 TID-os9okq0ko LdapGroupSyncWorker JID-3de7ecb87e5cc728c008ced0 INFO: start
2018-01-15_17:01:53.79301 2018-01-15T17:01:53.792Z 36325 TID-os9okq0ko LdapGroupSyncWorker JID-3de7ecb87e5cc728c008ced0 INFO: Started LDAP group sync for group AValidLDAPGroup (2098)
2018-01-15_17:01:54.45672 2018-01-15T17:01:54.456Z 36325 TID-os9okq0ko LdapGroupSyncWorker JID-3de7ecb87e5cc728c008ced0 INFO: Adding dead LdapGroupSyncWorker job 3de7ecb87e5cc728c008ced0
2018-01-15_17:01:54.55017 2018-01-15T17:01:54.549Z 36325 TID-os9okq0ko LdapGroupSyncWorker JID-3de7ecb87e5cc728c008ced0 INFO: fail: 1.792 sec
2018-01-15_17:01:54.55028 2018-01-15T17:01:54.550Z 36325 TID-os9okq0ko WARN: {"context":"Job raised exception","job":{"class":"LdapGroupSyncWorker","args":[[2098],"ldapmain"],"retry":3,"queue":"ldap_group_sync","jid":"3de7ecb87e5cc728c008ced0","created_at":1516035523.765594,"enqueued_at":1516035712.5679872,"error_message":"undefined method `<' for nil:NilClass","error_class":"NoMethodError","failed_at":1516035523.7869868,"retry_count":3,"retried_at":1516035714.4560423},"jobstr":"{\"class\":\"LdapGroupSyncWorker\",\"args\":[[2098],\"ldapmain\"],\"retry\":3,\"queue\":\"ldap_group_sync\",\"jid\":\"3de7ecb87e5cc728c008ced0\",\"created_at\":1516035523.765594,\"enqueued_at\":1516035712.5679872,\"error_message\":\"undefined method `<' for nil:NilClass\",\"error_class\":\"NoMethodError\",\"failed_at\":1516035523.7869868,\"retry_count\":2,\"retried_at\":1516035589.6277041}"}
2018-01-15_17:01:54.55040 2018-01-15T17:01:54.550Z 36325 TID-os9okq0ko WARN: /opt/gitlab/embedded/service/gitlab-rails/ee/lib/ee/gitlab/ldap/sync/group.rb:56:in `fail_stuck_group'
2018-01-15_17:01:54.55042 /opt/gitlab/embedded/service/gitlab-rails/ee/lib/ee/gitlab/ldap/sync/group.rb:45:in `ldap_sync_ready?'
2018-01-15_17:01:54.55042 /opt/gitlab/embedded/service/gitlab-rails/ee/lib/ee/gitlab/ldap/sync/group.rb:32:in `execute'
2018-01-15_17:01:54.55042 /opt/gitlab/embedded/service/gitlab-rails/app/workers/ldap_group_sync_worker.rb:26:in `sync_group'
2018-01-15_17:01:54.55043 /opt/gitlab/embedded/service/gitlab-rails/app/workers/ldap_group_sync_worker.rb:20:in `block in sync_groups'
2018-01-15_17:01:54.55043 /opt/gitlab/embedded/service/gitlab-rails/app/workers/ldap_group_sync_worker.rb:20:in `sync_groups'
2018-01-15_17:01:54.55043 /opt/gitlab/embedded/service/gitlab-rails/app/workers/ldap_group_sync_worker.rb:12:in `block in perform'
2018-01-15_17:01:54.55044 /opt/gitlab/embedded/service/gitlab-rails/ee/lib/ee/gitlab/ldap/sync/proxy.rb:14:in `block in open'
2018-01-15_17:01:54.55044 /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/ldap/adapter.rb:14:in `block in open'
2018-01-15_17:01:54.55044 /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/net-ldap-0.16.0/lib/net/ldap.rb:644:in `block in open'
2018-01-15_17:01:54.55045 /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/net-ldap-0.16.0/lib/net/ldap.rb:716:in `block in open'
2018-01-15_17:01:54.55045 /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/net-ldap-0.16.0/lib/net/ldap/instrumentation.rb:19:in `instrument'
2018-01-15_17:01:54.55045 /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/net-ldap-0.16.0/lib/net/ldap.rb:711:in `open'
2018-01-15_17:01:54.55045 /opt/gitlab/embedded/lib/ruby/gems/2.3.0/gems/net-ldap-0.16.0/lib/net/ldap.rb:644:in `open'
2018-01-15_17:01:54.55045 /opt/gitlab/embedded/service/gitlab-rails/lib/gitlab/ldap/adapter.rb:13:in `open'
2018-01-15_17:01:54.55046 /opt/gitlab/embedded/service/gitlab-rails/ee/lib/ee/gitlab/ldap/sync/proxy.rb:13:in `open'
2018-01-15_17:01:54.55046 /opt/gitlab/embedded/service/gitlab-rails/app/workers/ldap_group_sync_worker.rb:11:in `perform'