Only return found LDAP groups for processing

lib/gitlab/ldap/access.rb

@@ -103,7 +103,7 @@ module Gitlab
       def ldap_groups
         @ldap_groups ||= ::LdapGroupLink.distinct(:cn).pluck(:cn).map do |cn|
           Gitlab::LDAP::Group.find_by_cn(cn, adapter)
-        end
+        end.compact
       end
 
       # returns a collection of cn strings to which the user has access

spec/lib/gitlab/ldap/access_spec.rb

@@ -194,11 +194,20 @@ objectclass: top
 objectclass: posixGroup
 })
     end
+
     it "returns an interator of LDAP Groups" do
       ::LdapGroupLink.create cn: 'example', group_access: Gitlab::Access::DEVELOPER, group_id: 42
       Gitlab::LDAP::Adapter.any_instance.stub(:group) { Gitlab::LDAP::Group.new(ldap_group_1) }
+
       expect(access.ldap_groups.first).to be_a Gitlab::LDAP::Group
     end
+
+    it "only returns found ldap groups" do
+      ::LdapGroupLink.create cn: 'example', group_access: Gitlab::Access::DEVELOPER, group_id: 42
+      Gitlab::LDAP::Group.stub(find_by_cn: nil) # group not found
+
+      expect(access.ldap_groups).to be_empty
+    end
   end
 
   describe :cns_with_access do