Document behaviour of null errorMessage attribute

[edwardhorsford]

Talking with @hannalaakso yesterday, we realised the design system docs could be clearer that a false or null errorMessage would be ignored.

My team had interpreted the docs that if the errorMessage attribute were provided, macros would output error styling. It's easier for us to provide a false / null errorMessage attribute than it is to conditionally provide the whole thing.

Related: it would possibly be good to extend this behaviour to empty strings for errorMessage, which might match recent work where empty arrays were ignored.