We simply implement ToCss for Option<T>, printing nothing if the value is None,
and we then use SequenceWriter to skip writing of separators around empty parts.
For now, only impls for types like in style::values::generics can be derived.
This also needed a few ToComputedValueAsSpecified impls that I would like to
replace by some #[to_computed_value(clone)] attribute, but I think it is ok
to keep it like this for now.
