Why the AMP sitemap is empty?
If you are using JAmp along with JSitemap to generate a specific AMP XML sitemap for your Joomla website, after enabling it in the JSitemap component configuration you have to activate the AMP sitemap on a data source basis.
Obviously having AMP on a website is optional, thus JSitemap by default doesn't include any type of content or link in the AMP sitemap generating it as an empty XML feed.
Based on components for which you have enabled an AMP version in the JAmp settings, you can enable the following parameter in the corresponding data source to have those links included in the AMP sitemap:
Obviously ensure that the AMP sitemap is enabled in the component configuration.