Official answer. Followed by Night is a special exception, as are cards targeting "all" of something. In that case, "all" could be 0. Otherwise, you have to choose an effect that you can resolve if possible.
When resolving any When Revealed effect that gives you a choice between two effects, you must choose an effect that you can fully resolve. Followed by Night is an odd example because you can always resolve either effect. That’s because it targets either ‘all allies’ or ‘all enemies’ and the word ‘all’ can include 0.
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Cheers,
Caleb