BUG #8398: to_json(''::hstore) gives invalid JSON

The following bug has been logged on the website:

Bug reference: 8398
Logged by: Kevin Martin
Email address: martin(dot)kevin(at)gmail(dot)com
PostgreSQL version: 9.3rc1
Operating system: Ubuntu 10.04
Description:

to_json() returns an invalid JSON value when called on an empty hstore
value. The first two statements below behave as expected, the third (with an
empty hstore) fails:

config_test=# SELECT to_json(q) FROM (SELECT '' AS v) AS q;
to_json
----------
{"v":""}
(1 row)

config_test=# SELECT to_json(q) FROM (SELECT ARRAY[]::INT[] AS v) AS q;
to_json
----------
{"v":[]}
(1 row)

config_test=# SELECT to_json(q) FROM (SELECT ''::HSTORE AS v) AS q;
to_json
---------
{"v":}
(1 row)

The returned JSON will fail to be parsed by a conformant JSON parser.