Merge forwardport of #12328 to 2.3-develop branch

Applied pull request patch https://github.com/magento/magento2/pull/12328.patch (created by @RomaKis) based on commit(s):
  1. 147f638

Fixed GitHub Issues in 2.3-develop branch:
  - #9742: Default welcome message returns after being deleted (reported by @robgt)

app/code/Magento/Theme/Model/Design/Config/Storage.php

@@ -87,10 +87,13 @@ public function load($scope, $scopeId)
                 $scopeId,
                 $fieldData->getFieldConfig()
             );
-            if ($value !== null) {
-                $fieldData->setValue($value);
+
+            if ($value === null) {
+                $value = '';
             }
+            $fieldData->setValue($value);
         }
+
         return $designConfig;
     }
 

dev/tests/integration/testsuite/Magento/Theme/Model/Design/ConfigTest.php

@@ -0,0 +1,40 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+
+namespace Magento\Theme\Model\Design;
+
+/**
+ * Test for \Magento\Theme\Model\Design\Config\Storage.
+ */
+class ConfigTest extends \PHPUnit\Framework\TestCase
+{
+    /**
+     * @var \Magento\Theme\Model\Design\Config\Storage
+     */
+    private $storage;
+
+    protected function setUp()
+    {
+        $this->storage = \Magento\TestFramework\Helper\Bootstrap::getObjectManager()->create(
+            \Magento\Theme\Model\Design\Config\Storage::class
+        );
+    }
+
+    /**
+     * Test design/header/welcome if it is saved in db as empty(null) it should be shown on backend as empty.
+     *
+     * @magentoDataFixture Magento/Theme/_files/config_data.php
+     */
+    public function testLoad()
+    {
+        $data = $this->storage->load('stores', 1);
+        foreach ($data->getExtensionAttributes()->getDesignConfigData() as $configData) {
+            if ($configData->getPath() == 'design/header/welcome') {
+                $this->assertSame('', $configData->getValue());
+            }
+        }
+    }
+}

dev/tests/integration/testsuite/Magento/Theme/_files/config_data.php

@@ -0,0 +1,19 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+
+use Magento\Config\Model\Config\Factory;
+use Magento\TestFramework\Helper\Bootstrap;
+
+$objectManager = Bootstrap::getObjectManager();
+
+/** @var Factory $configFactory */
+$configFactory = $objectManager->create(Factory::class);
+/** @var \Magento\Config\Model\Config $config */
+$config = $configFactory->create();
+$config->setScope('stores');
+$config->setStore('default');
+$config->setDataByPath('design/header/welcome', null);
+$config->save();

dev/tests/integration/testsuite/Magento/Theme/_files/config_data_rollback.php

@@ -0,0 +1,13 @@
+<?php
+/**
+ * Copyright © Magento, Inc. All rights reserved.
+ * See COPYING.txt for license details.
+ */
+
+$objectManager = \Magento\TestFramework\Helper\Bootstrap::getObjectManager();
+/** @var \Magento\Framework\App\ResourceConnection $resource */
+$resource = $objectManager->get(\Magento\Framework\App\ResourceConnection::class);
+$connection = $resource->getConnection();
+$tableName = $resource->getTableName('core_config_data');
+
+$connection->query("DELETE FROM $tableName WHERE path = 'design/header/welcome';");