External Links, Navigation, and the Sitemap in Eleventy

I'm still getting to know Eleventy and slowly adding things to the site. The first order of business was to add a link to my social media account to the menu bar. The menu bar is powered by the Navigation plugin. According to the documentation, adding permalink: false to the template will keep a file from being created in the generated output.

---
eleventyNavigation:
  key: Label
  url: https://external-domain.tld
  order: 4
permalink: false
---

However, as soon as I added the permalink line, there was an error generating the sitemap. The documentation on permalinks confirms that no output file will be created, but also notes it will still be included in collections. As you can see in the initial template for generating the sitemap (I'm using eleventy-base-blog as the basis of my site), it iterates over collections.all.

---
permalink: /sitemap.xml
eleventyExcludeFromCollections: true
---
<?xml version="1.0" encoding="utf-8"?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9" xmlns:xhtml="http://www.w3.org/1999/xhtml">
{%- for page in collections.all %}
	{% set absoluteUrl %}{{ page.url | htmlBaseUrl(metadata.url) }}{% endset %}
	<url>
		<loc>{{ absoluteUrl }}</loc>
		<lastmod>{{ page.date | htmlDateString }}</lastmod>
	</url>
{%- endfor %}
</urlset>

When it gets to the external link where permalink is set to false, the filter htmlBaseUrl throws an error. Since the external link shouldn't be in the sitemap anyway, testing the truthiness of page.url before creating the entry does the trick.

---
permalink: /sitemap.xml
eleventyExcludeFromCollections: true
---
<?xml version="1.0" encoding="utf-8"?>
<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9" xmlns:xhtml="http://www.w3.org/1999/xhtml">
{%- for page in collections.all %}
	{%- if page.url %}
	{% set absoluteUrl %}{{ page.url | htmlBaseUrl(metadata.url) }}{% endset %}
	<url>
		<loc>{{ absoluteUrl }}</loc>
		<lastmod>{{ page.date | htmlDateString }}</lastmod>
	</url>
	{%- endif %}
{%- endfor %}
</urlset>