If Sunfall resolves but doesn't exile any creatures, you'll incubate 0, creating an Incubator token but putting no +1/+1 counters on it. It will still have the ability to transform, but you might need to figure out how it's going to survive first.

Sunfall : règles, fonctionnement et interactions (MTG)

Sunfall is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

Exile all creatures. Incubate X, where X is the number of creatures exiled this way. (Create an Incubator token with X +1/+1 counters on it and ": Transform this token." It transforms into a 0/0 Phyrexian artifact creature.)

How it works : Sunfall

Original English text — Wizards of the Coast / Scryfall

#1April 14, 2023· Wizards of the Coast
If Sunfall resolves but doesn't exile any creatures, you'll incubate 0, creating an Incubator token but putting no +1/+1 counters on it. It will still have the ability to transform, but you might need to figure out how it's going to survive first.
No other card is named in this ruling.