Customer related values are NULL for guests

Author: Nazar65

app/code/Magento/Quote/Model/QuoteManagement.php

@@ -354,6 +354,12 @@ public function placeOrder($cartId, PaymentInterface $paymentMethod = null)
         if ($quote->getCheckoutMethod() === self::METHOD_GUEST) {
             $quote->setCustomerId(null);
             $quote->setCustomerEmail($quote->getBillingAddress()->getEmail());
+            $quote->setCustomerFirstname($quote->getBillingAddress()->getFirstname());
+            $quote->setCustomerLastname($quote->getBillingAddress()->getLastname());
+
+            if ($quote->getBillingAddress()->getMiddlename()) {
+                $quote->setCustomerMiddlename($quote->getBillingAddress()->getMiddlename());
+            }
             $quote->setCustomerIsGuest(true);
             $quote->setCustomerGroupId(\Magento\Customer\Api\Data\GroupInterface::NOT_LOGGED_IN_ID);
         }

app/code/Magento/Quote/Test/Unit/Model/QuoteManagementTest.php

@@ -644,7 +644,7 @@ public function testPlaceOrderIfCustomerIsGuest()
 
         $addressMock = $this->createPartialMock(\Magento\Quote\Model\Quote\Address::class, ['getEmail']);
         $addressMock->expects($this->once())->method('getEmail')->willReturn($email);
-        $this->quoteMock->expects($this->once())->method('getBillingAddress')->with()->willReturn($addressMock);
+        $this->quoteMock->expects($this->any())->method('getBillingAddress')->with()->willReturn($addressMock);
 
         $this->quoteMock->expects($this->once())->method('setCustomerIsGuest')->with(true)->willReturnSelf();
         $this->quoteMock->expects($this->once())