Update ImportMapsManager.php

Closes #3501109

src/ImportMapsManager.php

@@ -27,7 +27,7 @@ final class ImportMapsManager extends DefaultPluginManager implements PluginMana
    * @param \Drupal\Core\Extension\ModuleExtensionList $moduleExtensionList
    *   Module extension list.
    */
-  public function __construct(ModuleHandlerInterface $module_handler, CacheBackendInterface $cache_backend, protected ModuleExtensionList $moduleExtensionList) {
+  public function __construct(ModuleHandlerInterface $module_handler, CacheBackendInterface $cache_backend, protected ?ModuleExtensionList $moduleExtensionList) {
     $this->alterInfo('importmaps');
     $this->moduleHandler = $module_handler;
     $this->setCacheBackend($cache_backend, 'importmaps', ['importmaps']);