TODO: add test

[thysultan]

Reminder to devise a test case for https://github.com/thysultan/stylis.js/commit/bbba67bae85c09600cc28e223eb804e1d779ffa6 so that we don't re-introduce a possible breaking change on that api contract in the future.

Issue

At-rules where unnecessarily leaking their .length property to their children. Causing upstream projects that auto detect using the length property for pseudo selector presence to break.