Set e-mail address for the PayPal guest quote's customer model

This change sets an e-mail address on the customer model assigned to the
guest quote prepared for further processing by PayPal module. It
circumvents inability to place a guest order with PayPal Express when
Automatic Asignment to Customer Group is enabled.

Fixes #25399

Author: xylodev

app/code/Magento/Paypal/Model/Express/Checkout.php

@@ -1159,6 +1159,9 @@ protected function prepareGuestQuote()
             ->setCustomerEmail($quote->getBillingAddress()->getEmail())
             ->setCustomerIsGuest(true)
             ->setCustomerGroupId(\Magento\Customer\Model\Group::NOT_LOGGED_IN_ID);
+        if ($quote->getCustomer()->getEmail() === null) {
+            $quote->getCustomer()->setEmail($quote->getBillingAddress()->getEmail());
+        }
         return $this;
     }
 }