Correct LDAP paging example (#2910)

empty('0') is true in PHP, but $cookie is an opaque string.
strlen should work in all the same places, but also for '0' (e.g. python-ldapserver returns this for the first response to every query).

reference/ldap/examples.xml

@@ -198,7 +198,7 @@ do {
         $cookie = '';
     }
     // Empty cookie means last page
-} while (!empty($cookie));
+} while (strlen($cookie) > 0);
 ?>
 ]]>
    </programlisting>