MAGETWO-58265: [Github][Cloud][Customer]Fix Varnish X-Header

- Added backward compatible workaround in observers

Author: Andrey Konosov

app/code/Magento/CacheInvalidate/Observer/InvalidateVarnishObserver.php

@@ -51,8 +51,11 @@ public function __construct(
      */
     public function execute(\Magento\Framework\Event\Observer $observer)
     {
+        $object = $observer->getEvent()->getObject();
+        if (!is_object($object)) {
+            return;
+        }
         if ($this->config->getType() == \Magento\PageCache\Model\Config::VARNISH && $this->config->isEnabled()) {
-            $object = $observer->getEvent()->getObject();
             $bareTags = $this->tagResolver->getTags($object);
 
             $tags = [];

app/code/Magento/PageCache/Observer/FlushCacheByTags.php

@@ -49,8 +49,12 @@ public function __construct(\Magento\PageCache\Model\Config $config, \Magento\Fr
      */
     public function execute(\Magento\Framework\Event\Observer $observer)
     {
+        $object = $observer->getEvent()->getObject();
+        if (!is_object($object)) {
+            return;
+        }
+
         if ($this->_config->getType() == \Magento\PageCache\Model\Config::BUILT_IN && $this->_config->isEnabled()) {
-            $object = $observer->getEvent()->getObject();
             $tags = $this->getTagResolver()->getTags($object);
 
             if (!empty($tags)) {
@@ -62,6 +66,7 @@ public function execute(\Magento\Framework\Event\Observer $observer)
     /**
      * TODO: Workaround to support backwards compatibility, will rework to use Dependency Injection in MAGETWO-49547
      *
+     *
      * @return \Magento\PageCache\Model\Cache\Type
      */
     private function getCache()