Merge forwardport of #12738 to 2.3-develop branch

Applied pull request patch https://github.com/magento/magento2/pull/12738.patch (created by @xpoback) based on commit(s):
  1. 6b9c39c

Fixed GitHub Issues in 2.3-develop branch:
  - #12719: Welcome message is shown with customer's first and last names after confirming account (reported by @alena-marchenko)

Author: magento-engcom-team

app/code/Magento/Theme/view/frontend/templates/html/header.phtml

@@ -15,7 +15,7 @@ $welcomeMessage = $block->getWelcome();
     case 'welcome': ?>
         <li class="greet welcome" data-bind="scope: 'customer'">
             <!-- ko if: customer().fullname  -->
-            <span data-bind="text: new String('<?= $block->escapeHtml(__('Welcome, %1!', '%1')) ?>').replace('%1', customer().firstname)">
+            <span data-bind="text: new String('<?= $block->escapeHtml(__('Welcome, %1!', '%1')) ?>').replace('%1', customer().fullname)">
             </span>
             <!-- /ko -->
             <!-- ko ifnot: customer().fullname  -->

dev/tests/functional/tests/app/Magento/Checkout/Test/Constraint/AssertCartPerCustomer.php

@@ -53,7 +53,10 @@ public function processAssert(
                     ['customer' => $customer]
                 )->run();
                 \PHPUnit_Framework_Assert::assertEquals(
-                    sprintf(self::WELCOME_MESSAGE, $customer->getFirstname()),
+                    sprintf(
+                        self::WELCOME_MESSAGE,
+                        $customer->getFirstname() . ' ' . $customer->getLastname()
+                    ),
                     $cmsIndex->getLinksBlock()->getWelcomeText(),
                     'Customer welcome message is wrong.'
                 );