Fixed assignment of the guest customer to the guest group.

This commit fixes the case when guest fills in valid VAT number,
then returns to the shipping step and removes VAT from the address.

Author: vovayatsyuk

app/code/Magento/Quote/Observer/Frontend/Quote/Address/CollectTotalsObserver.php

@@ -124,7 +124,7 @@ public function execute(\Magento\Framework\Event\Observer $observer)
             );
         }
 
-        if ($groupId) {
+        if ($groupId !== null) {
             $address->setPrevQuoteCustomerGroupId($quote->getCustomerGroupId());
             $quote->setCustomerGroupId($groupId);
             $this->customerSession->setCustomerGroupId($groupId);