gdesktopappinfo: Add missing annotations

Add missing (allow-none) annotations to
g_desktop_app_info_launch_uris_as_manager(). Tested with

  python -c "from gi.repository import GLib, Gio; Gio.DesktopAppInfo.new('gcalctool.desktop').launch_uris_as_manager([], None, GLib.SpawnFlags.SEARCH_PATH|GLib.SpawnFlags.STDOUT_TO_DEV_NULL, None, None, None, None)"

which is necessary to do a launch_uris() without leaking stdout.

https://launchpad.net/bugs/1259721