Issue #3471741 by mstrelan, bbrala, kristiaanvandeneynde: Fix null $cid in CacheCollector classes

(cherry picked from commit 85aa92b0)

core/lib/Drupal/Core/Asset/LibraryDiscoveryCollector.php

@@ -43,7 +43,7 @@ class LibraryDiscoveryCollector extends CacheCollector {
    */
   public function __construct(CacheBackendInterface $cache, LockBackendInterface $lock, LibraryDiscoveryParser $discovery_parser, ThemeManagerInterface $theme_manager) {
     $this->themeManager = $theme_manager;
-    parent::__construct(NULL, $cache, $lock, ['library_info']);
+    parent::__construct('', $cache, $lock, ['library_info']);
 
     $this->discoveryParser = $discovery_parser;
   }
@@ -52,7 +52,7 @@ public function __construct(CacheBackendInterface $cache, LockBackendInterface $
    * {@inheritdoc}
    */
   protected function getCid() {
-    if (!isset($this->cid)) {
+    if ($this->cid === '') {
       $this->cid = 'library_info:' . $this->themeManager->getActiveTheme()->getName();
     }
 
@@ -164,7 +164,7 @@ protected function applyLibrariesExtend($extension, $library_name, $library_defi
    */
   public function reset() {
     parent::reset();
-    $this->cid = NULL;
+    $this->cid = '';
   }
 
 }

core/lib/Drupal/Core/Menu/MenuActiveTrail.php

@@ -42,7 +42,7 @@ class MenuActiveTrail extends CacheCollector implements MenuActiveTrailInterface
    *   The lock backend.
    */
   public function __construct(MenuLinkManagerInterface $menu_link_manager, RouteMatchInterface $route_match, CacheBackendInterface $cache, LockBackendInterface $lock) {
-    parent::__construct(NULL, $cache, $lock);
+    parent::__construct('', $cache, $lock);
     $this->menuLinkManager = $menu_link_manager;
     $this->routeMatch = $route_match;
   }
@@ -53,7 +53,7 @@ public function __construct(MenuLinkManagerInterface $menu_link_manager, RouteMa
    * @see ::getActiveTrailIds()
    */
   protected function getCid() {
-    if (!isset($this->cid)) {
+    if ($this->cid === '') {
       $route_parameters = $this->routeMatch->getRawParameters()->all();
       ksort($route_parameters);
       $this->cid = 'active-trail:route:' . $this->routeMatch->getRouteName() . ':route_parameters:' . serialize($route_parameters);