Allow no interface (for active agents)

[lug-gh]

Hi, currently it is required that you define an interface. But if you use the Zabbix agent as "Active", then no interface is needed, because the agent has no open port.

Would it be possible to customize the provider to use active agents?
I can set an interface during the creation by terraform, if I delete it afterwards in Zabbix I have the desired result, but it is manual work. I also noticed that the interface is not created again after terraform apply, other changes made in Zabbix (e.g. the name) are recognized and overwritten by terraform.