Ensure that users_nothing_done is created as an array

This is likely only going to generate an error on the first time you
sync from LDAP, but still, let's not spit an unncessary error out.

inc/drivers_ldap.php

@@ -696,6 +696,8 @@ function sync_LDAP(){
   $users_to_deactivate = array_diff($db_users,$ldap_users);
   // users present in ldap and in the db
   $users_to_update = array_intersect($db_users,$ldap_users);
+  // users not modified
+  $users_nothing_done = array();
 
   // creation of all users;
   if ( sizeof($users_to_create) ) {