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.
The number of snow Swamps you control and the number of times Withering Wisps's ability has been activated so far that turn are checked each time you attempt to activate the ability.
If all creatures leave the battlefield during a turn's End step (after any "at the beginning of the end step" triggers would have triggered that turn), Withering Wisps will remain on the battlefield because the trigger condition has already passed.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. NéNé Thomas
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