Differentiate between `null` and `undefined`

[bartveneman]

Working on https://www.projectwallace.com/ast-explorer. When cloning or debugging a node it should be clear that there's a difference between some properties:

• is_vendor_prefixed should be true | false | undefined (undefined for nodes like atrule, stylesheet etc.)
• is_important should be true | false | undefined (only boolean when node is declaration)
• prelude should be string | null | undefined. Can be string | null for atrules, undefined otherwise

etc. etc. for value, name

Cloning should omit undefined values.