Enforce access rules does not work on search pages

In the db_rewrite_sql we have this code:

  // In any of the following cases, do not enforce any rules.
  if (!$admin_force || empty($query) || $primary_field != 'nid' || !user_access('administer nodes') || domain_grant_all()) {
    return;
  }

The empty($query) was a safeguard against, well, empty queries, but node searching invokes db_rewrite_sql() without passing a $query, so we have to remove that if searching is to work properly.

Testing needed to see if this introduces any errors.