Read this great article Stop checking for NULL pointers! It mentions that there are many other invalid pointers than NULL, which is the same idea I had recently with regard to foreign keys in databases.
The configuration file is in
/usr/local/etc/zabbix/zabbix_agentd.conf. If you make changes you can restart with:
# launchctl stop com.zabbix.zabbix_agentd # launchctl start com.zabbix.zabbix_agentd