In its current state this script will likely never produce a result for an idle query.
Currently it is considering only query_start:
age(now(), query_start)
I propose coalesce with transaction and query start times:
age(now(), coalesce( xact_start, query_start))
 

I have attached a fixed version, but can also do a PR if needed.

Steps to reproduce:

1. run the sql script 
   /var/lib/zabbix/postgresql/pgsql.transactions.sql

Result:

{"idle":0,"active":0.903109,"waiting":-0.009843,"prepared":0,"xmin_age":25}

Expected:

{"idle":<SOME NUMBER>,"active":0.903109,"waiting":-0.009843,"prepared":0,"xmin_age":25}