Nolink parent menu items: missing pointer cursor and click navigates to `/undefined`

Problem/Motivation

When a main-navigation parent menu item is a nolink (a parent that has children but no URL — Drupal's <nolink> route), bs_base renders it as a plain <span>:

<span class="nav-link"><span class="label dropdown-toggle">Der Verband</span></span>

Because this is a <span> with no href, two problems follow:

1. Wrong cursor. The <span> shows the default text caret on hover, so the clickable dropdown toggle does not read as clickable (a real a.nav-link would show a pointer).
2. Click navigates to /undefined. With navbar_opened_submenu_behavior: visit, clicking an already-open toggle fires hide.bs.dropdown, and Drupal.bs_base.navbar.followLink() runs window.location = link.href. A nolink <span> has no href, so link.href is undefined and the browser navigates to /undefined (404).

Steps to reproduce

1. A second-level-dropdown navbar with navbar_opened_submenu_behavior: visit.
2. A top-level menu item using <nolink> (children, no URL).
3. Hover the parent → text caret instead of a pointer.
4. Open its dropdown, then click the parent again → navigates to /undefined.

Proposed resolution

1. Drupal.bs_base.navbar.followLink() — bail out when the element has no href (a nolink toggle has no destination to follow).
2. CSS — .navbar-nav span.nav-link { cursor: pointer; } so nolink toggles read as clickable.

Both changes touch js/navbar.js and sass/components/partials/_navbar.scss (see the MR).