open-uri: Don't ever pass a list of NULLs to g_app_info_launch_uris()
open-uri: Don't ever pass a list of NULLs to g_app_info_launch_uris()

Add extra checks to never do this when registering the URI with the
documents portal (for sandboxed handler) fails for any reason, and
report to the sandboxed caller accordingly when that happens.

https://github.com/flatpak/xdg-desktop-portal/issues/134

flatpak/xdg-desktop-portal,flatpak/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,flatpak/xdg-desktop-portal,flatpak/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal,matthiasclasen/xdg-desktop-portal