MDL-55700 theme_noname: Allow text nodes in breadcrumb

The new theme was showing all nodes in the breadcrumb as links even when there is no
valid link.

Part of MDL-55071

lib/navigationlib.php

@@ -592,6 +592,15 @@ class navigation_node implements renderable {
         }
     }
 
+    /**
+     * Used to easily determine if this link in the breadcrumbs has a valid action/url.
+     *
+     * @return boolean
+     */
+    public function has_action() {
+        return !empty($this->action);
+    }
+
     /**
      * Gets the CSS class to add to this node to describe its type
      *