[css-typed-om-1] Should probably be more explicit about 0 handling in "invert"

[bzbarsky]

https://drafts.css-houdini.org/css-typed-om-1/#cssmath-invert step 2 substep 1 says:

If this’s value internal slot is set to 0, throw a RangeError.

It's not clear whether this means "is set to a double that is == 0" or actually "is set to 0". In particular, it's not clear whether -0 should throw a RangeError. I expect it should and this should be made clear by doing s/0/0 or -0/.