The default value of the --linux-menu-group option is "Unknown". The value of this option is used to set the "Categories" key in a corresponding .desktop file.

However, "Unknown" is not a valid "Categories" key value.
The list of valid values is at [1] and [2].

Validation .desktop files created in jpackage with "desktop-file-validate" command fails:
---
foo.desktop: error: value "Unknown" for key "Categories" in group "Desktop Entry" contains an unregistered value "Unknown"; values extending the format should start with "X-"
foo.desktop: hint: value "Unknown" for key "Categories" in group "Desktop Entry" does not contain a registered main category; application might only show up in a "catch-all" section of the application menu
---

[1] https://specifications.freedesktop.org/menu-spec/latest/category-registry.html#main-category-registry
[2] https://specifications.freedesktop.org/menu-spec/latest/additional-category-registry.html

Need a valid default value for "Categories" key.