Issue #3486949: Add getMenus() method to support subclassing

Closes #3486949

src/MenuBasedBreadcrumbBuilder.php

@@ -205,7 +205,7 @@ class MenuBasedBreadcrumbBuilder implements BreadcrumbBuilderInterface {
 
     // Check each selected menu, in turn, until a menu or taxonomy match found:
     // then cache its state for building & caching in build() and exit.
-    $menus = $this->config->get('menu_breadcrumb_menus');
+    $menus = $this->getMenus();
     uasort($menus, function ($a, $b) {
       return SortArray::sortByWeightElement($a, $b);
     });
@@ -437,6 +437,16 @@ class MenuBasedBreadcrumbBuilder implements BreadcrumbBuilderInterface {
     }
   }
 
+  /**
+   * Retrieve menus from configuration.
+   *
+   * @return array
+   *   Configuration menus.
+   */
+  protected function getMenus(): array {
+    return $this->config->get('menu_breadcrumb_menus') ?: [];
+  }
+
   /**
    * The getter function for $menuName property.
    *