Configuring an interface without IP is possible but lead to inconsistent behavior

[nqb]

Describe the bug
You can configure an interface without any IP address using GUI (Configurator or Interfaces menu).
Results:

• interface is created on OS
• interface is defined in pf.conf like this:

[interface eth0.100]
mask=255.255.255.0
type=dhcp-listener

(only if you set a type, otherwise interface is not added to /etc/network/interfaces)

• nothing is written in /etc/network/interfaces

If you reboot, this new interface can be down because nothing tell the OS that this interface should be up (nothing is written in /etc/network/interfaces).

Also see #5674.

To Reproduce
Steps to reproduce the behavior:

1. Configure an interface without any IP address and with or without a type

=> API and frontend let your perform this change
=> Interface is down or doesn't exist anymore

Expected behavior
Write network definition in OS files to be sure pf.conf and network OS files are consistent.

Additional context

• Behavior noticed on 12.0.0 and 12.1.0 setup on Debian (didn't look on EL8)
• For me, it makes sense to have DHCP listener interfaces without any IP because they are here only to listen traffic at L2/L3