getCampaignId errors when no session

Problem/Motivation

If you don't have a session (e.g. running via cron), getCampaignId errors.

Proposed resolution

This will fix it using PHP8 and later.

MailchimpEcommerceHelper.php
line 110:
$session_campaign = $this->request->getSession()->get('mc_cid', '');
should change to
$session_campaign = $this->request->getSession()?->get('mc_cid', '');