gdesktopappinfo: Don't crash if we don't have a desktop filename

gdesktopappinfo: Don't crash if we don't have a desktop filename

If code creates a GDesktopAppInfo via g_desktop_app_info_new_from_keyfile(),
we'd try to send a NULL pointer down into GVariant.

Since in this case we don't have a filename, just send the empty
string.  In the future we should either:

1) Change panel to use g_desktop_app_info_new_from_filename(), and
   take the hit of parsing the file twice.
2) Add a g_key_file_get_origin_filename()
3) Add g_desktop_app_info_new_from_keyfile_and_name()

https://bugzilla.gnome.org/show_bug.cgi?id=638838