Document convention around returning `null` for known-but-undefined values

[kadamwhite]

See WP-API/WP-API#2657 for the relating discussion: One of the conventions of the WordPress REST API is that if a value is undefined for a specific key that should be part of a response, the API should provide null as the value for that key. Receiving a null value therefore means, "we know and understand this field, and it lacks a value".