You don't choose whether to sacrifice an artifact or which artifact you're sacrificing until the ability resolves. Once you decide to sacrifice an artifact (or not), it's too late for players to respond.
Kuldotha Flamefiend
If some of the targets become illegal for Kuldotha Flamefiend's ability, the original division of damage still applies, but the damage that would have been dealt to illegal targets isn't dealt at all. If all of the targets become illegal, you can't sacrifice an artifact.
You may choose zero targets. If you do, Kuldotha Flamefiend deals no damage, but you may sacrifice an artifact.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Raymond Swanland
Card data via Scryfall. Rarix is fan-made; not endorsed by the rights holders.
