gs-appstream: Always use first release description

When adding updates in gs_appstream_refine_app_updates(), there is unconditionally used the first release's description when there's only one release available in the appstream data, but when there are more versions, the first is skipped. The app passed to this function is in an updatable state, thus the appstream may contain the needed information. It might not be always accurate, because the Flatpak apps can be released multiple times in the repository (different commits), while having always the same 'release' information in the appstream data of that app, but having there at least some information might look better than "No description available".