It will be postponed. The legal precdence was set last season in Napoli-Juve, where Serie A doesnt overrule the law. If a heaalth authority bans a team from travelling, then the game is de facato postponed. If a team choose not to show up desspite being able to field 14 people (including primaavera), then I guess it'll be a forfeit.
I'm not sure how I feel about the latter yet. I think I ddisagree with it, I think they should be able to forfeit if there are that many aabsences with Covid. The Premiership has been quite sensible asa far as I can see for this, quite a few games have been postponed din the lsat couple of weeks.