maps don't work in dashboard without php-json

if a map is added to the dashboard, it does not work :

PHP Fatal error: Call to undefined function json_decode() in /srv/www/htdocs/zabbix/app/controllers/CControllerWidgetSysmapView.php on line 79, referer: http://192.168.56.10/zabbix/zabbix.php?action=dashboard.view&dashboardid=2

php-json is not listed as a requirement at https://www.zabbix.com/documentation/3.4/manual/installation/requirements#software , and frontend dependency check does not verify for it either.

this dependency should be either removed, or documented and verified in the dependency check