Why doesn't jsonb constructor from a NULL record behave like hstore constructor?
The workaround here is to construct the jsonb by proxy of jsonb_populate_record

SELECT jsonb_populate_record(
  null::my_type,
  '{}'
);


Or, in the event you're just trying to update the record type, you can do it directly with the one call.

SELECT jsonb_populate_record(
  null::my_type,
  jsonb_build_object('name', 'Evan')
);