“How many pigs are there?” And answering with “There are no pigs” use the noun “pigs” in the same way. They are referring to the “pig” category or kind. When answering knowing the actual count, it’s a specific number or token.

There are one pigs

And also it's not just zero, plural is for literally anything other than (positive) one: 2.3 pigs, -1 pigs, 0.2 pigs, etc.