https://www.zabbix.com/documentation/current/en/manual/config/triggers/trigger
It does not say what happen if we leave the field empty.

In fact, it will show there a last item value received, by default.
This might be unexpected, must be documented.

Confirmed for 7.0, older releases were not checked.
As an example, "Nutanix Prism Element by HTTP" official template has been used.