json_populate_record throws an error if asked to convert a JSON scalar
or array into a composite type.  jsonb_populate_record was returning
a record full of NULL fields instead.  It seems better to make it
throw an error for this case as well.

Nikita Glukhov

Discussion: https://postgr.es/m/fbd1d566-bba0-a3de-d6d0-d3b1d7c24ff2@postgrespro.ru