🍒 RemoteDesktop: Include clipboard_enabled and the restore data even when no screens are requested

The portal documentation says that the Start call should return at least three values, which are devices, clipboard_enabled and restore_token, regardless of whether screens are included in the portal request.

In particular this fixes persistence not working for apps that only request inputs and not screens like KDE Connect's remote input plugin (I have some WIP code for that here: psariputra/kdeconnect-kde@89ca5684)

CCBUG: 479013 (cherry picked from commit 356e7f9e)