At the beginning of the end step, if no creatures are on the battlefield, sacrifice Withering Wisps.
: Withering Wisps deals 1 damage to each creature and each player. Activate no more times each turn than the number of snow Swamps you control.
Withering Wisps’s triggered ability has an “intervening ‘if’ clause.” That means (1) the ability won’t trigger at all unless there are no creatures on the battlefield when a turn’s end step begins, and (2) the ability will do nothing if there are any creatures on the battlefield by the time the ability resolves.