Fix external links for sites with no pages (#1021)

Fix #1020

- Move the display of nav external links from `_includes/nav.html` to `_layouts/default.html`.
- Replace ` unless include.key` by `if site.nav_external_links`.
- Wrap the body of `if site.nav_external_links` in `<ul class="nav-list">…</ul>`.

To test this PR:

1.  Add to `_config.yml`:
    ```yaml
    defaults:
      -
        scope: {path: ""}
        values: {nav_exclude: true}
    ```

2.  Check that the only link to appear in the nav panel is external.

Co-authored-by: Matt Wang <matt@matthewwang.me>

_includes/nav.html

@@ -207,17 +207,6 @@
       </li>
     {%- endunless -%}
 {%- endfor -%}
-{%- unless include.key -%}
-  {%- assign nav_external_links = site.nav_external_links -%}
-  {%- for node in nav_external_links -%}
-        <li class="nav-list-item external">
-          <a href="{{ node.url | absolute_url }}" class="nav-list-link external">
-            {{ node.title }}
-            {% unless node.hide_icon %}<svg viewBox="0 0 24 24" aria-labelledby="svg-external-link-title"><use xlink:href="#svg-external-link"></use></svg>{% endunless %}
-          </a>
-        </li>
-  {%- endfor -%}
-{%- endunless -%}
 </ul>
 
 {%- comment -%}