Checks the values a create would write for one attribute against the editor's rules, and returns them in their normal form.
the attribute, in any case
the values, empty ones already removed
null when the editor does not know the attribute; otherwise { name, values }, or { error } naming what was wrong
{ name, values }
{ error }
Checks the values a create would write for one attribute against the editor's rules, and returns them in their normal form.