Re: [csswg-drafts] [css-values] Clarify that calc(0) is never a length (#4554)

I think the important spec quote is in https://drafts.csswg.org/css-values/#length-value:

> However, if a 0 could be parsed as either a `<number>` or a `<length>` in a property (such as line-height), it must parse as a `<number>`.

Then, when https://drafts.csswg.org/css-values/#calc-type-checking could parse `0` in `calc()` as either a `<number>` or `<length>`...

> - `<number>`
> - `<integer>`
>    the type is «[ ]» (empty map)
> - `<length>`
>    the type is «[ "length" → 1 ]»

...it must do it as a `<number>`. So it follows that `calc(0)` is not a length.

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