213 is fixed

As reported on https://bugzilla.mozilla.org/show_bug.cgi?id=1773624 and on several other duplicates, users trying to save a download to a CIFS remote share that has been mounted from Nautilus ends up with nothing happening.

Debugging has reveal that uri in https://github.com/flatpak/xdg-desktop-portal/blob/e3ca5567a2dbb5b763a383bba5ff1c0437a7e851/src/documents.c#L79 is okay, referring to a smb:// value, but the subsequent call returns a nullptr https://github.com/flatpak/xdg-desktop-portal/blob/e3ca5567a2dbb5b763a383bba5ff1c0437a7e851/src/documents.c#L80

I have been pointed to that this is on purpose and handled by https://github.com/flatpak/xdg-desktop-portal/blob/fe9c5a11a199c966b32f6b7327136782544b845e/src/xdg-desktop-portal.c#L380-L381

I have also been pointed that there is another upstream issue to fix the behavior and allow users to perform that flow: https://github.com/flatpak/xdg-desktop-portal/issues/213

Until that issue is fixed, the XDG portal UI should probably not show remote mount points it cannot handle, otherwise it is confusing users.