fix spec failures

app/models/ldap_group_link.rb

@@ -8,7 +8,7 @@ class LdapGroupLink < ActiveRecord::Base
   validates :cn, uniqueness: { scope: [:group_id, :provider] }, unless: :filter
   validates :filter, :group_access, :group_id, presence: true, unless: :cn
   validates :filter, uniqueness: { scope: [:group_id, :provider] }, unless: :cn
-  validates :filter, ldap_filter: true
+  validates :filter, ldap_filter: true, if: :filter
   validates :group_access, inclusion: { in: Gitlab::Access.all_values }
   validates :provider, presence: true