fixed empty system message on not refresheg page with previous system warrning

Author: Michal Malinowski

app/code/Magento/AdminNotification/Controller/Adminhtml/System/Message/ListAction.php

@@ -13,6 +13,10 @@ class ListAction extends \Magento\Backend\App\AbstractAction
     public function execute()
     {
         $severity = $this->getRequest()->getParam('severity');
+        $default = array(
+            'severity' => $severity,
+            'text' => 'All recent issues have been fixed. Please refresh the screen for an update.'
+            );
         $messageCollection = $this->_objectManager->get(
             'Magento\AdminNotification\Model\Resource\System\Message\Collection'
         );
@@ -23,6 +27,9 @@ public function execute()
         foreach ($messageCollection->getItems() as $item) {
             $result[] = ['severity' => $item->getSeverity(), 'text' => $item->getText()];
         }
+        if (empty($result)) {
+            $result[] = $default;
+        }
         $this->getResponse()->representJson(
             $this->_objectManager->get('Magento\Core\Helper\Data')->jsonEncode($result)
         );