✅ Fix failing isAnimationNone assertion in IE11

[rcebulko]

The animation-play-state property in IE11 is apparently known to be a bit funky, and returns an empty string for document.body.style.animationPlayState when the property remains unset through the CSS shorthand syntax. Since empty string isn't part of the allowed set defined for the assertion, the isAnimationNone fails for tests in IE11.

This PR adds the empty string to the accepted values for animation-play-state

Part of broad effort under #28152 to enable IE11 integration tests as blocking

Fixes failure:

[rcebulko]

@caroqliu requested you because I know this is UI territory, last person to touch most of this was Carlos many years ago, so I figured either you'd know what's up or you'd know who would.

[caroqliu]

Thanks for the heads up Ryan. Paging @alanorozco who's most likely to know better what relies on isAnimationNone and why.