The following bug has been logged on the website:
Bug reference: 15763
Logged by: Jacob Crell
Email address: jacobcrell(at)gmail(dot)com
PostgreSQL version: 9.6.8
Operating system: AWS RDS
Description:
SELECT '{"test":null}'::json->>'test' will return a null
SELECT '{"test":null}'::json->'test' will return a string 'null'
The 2nd option seems like it should also return a null.