MDL-4248 Fix error when updating user profiles and any of these field…

…s is empty

According to http://es2.php.net/manual/en/function.ldap-modify.php#43216 we
need to specify an array() value to delete an attribute's value, instead of an
empty string.

Merged from MOODLE_18_STABLE

auth/ldap/auth.php

@@ -1099,6 +1099,7 @@ function user_update($olduser, $newuser) {
                     }
 
                     $nuvalue = $textlib->convert($newuser->$key, 'utf-8', $this->config->ldapencoding);
+                    empty($nuvalue) ? $nuvalue = array() : $nuvalue;
                     $ouvalue = $textlib->convert($olduser->$key, 'utf-8', $this->config->ldapencoding);
 
                     foreach ($ldapkeys as $ldapkey) {