test should include isset in API contact.php

I get a php warning when importing contacts.
It's due to line 501 of civicrm/api/v3/Contact.php.

This is the original code:

$customValue = $params['contact_id'] ? CRM_Core_DAO::getFieldValue('CRM_Contact_DAO_Contact',
$params['contact_id'],
"{$key}{$greeting}_custom"
) : FALSE;

adding an isset() removes the php warning:

$customValue = isset($params['contact_id']) ? CRM_Core_DAO::getFieldValue('CRM_Contact_DAO_Contact',
$params['contact_id'],
"{$key}{$greeting}_custom"
) : FALSE;