API should send and accept null for undefined values #2696

See #2657 for the decision on null handling: null should be sent by the API whenever a value is unknown or not yet set.

A prime example of this is post_date_gmt, as described in #2303, which will be 0000-00-00 00:00:00 (an invalid date) for posts newly created via WP-Admin or via this API: if this value is sent to a client literally the API is providing an invalid value. null should be sent instead.

Correspondingly, null should be accepted by the API for post_date_gmt (and other fields that display this issue), so that a client can always turn the response for a resource around and send it back without further transformation: returning the API's output JSON as unchanged input should never trigger an error.