[nics] Error returned when optional field not set

[karencfv]

When adding a custom network interface I got an error when I didn't set the optional IP address field

[david-crespo]

Dupe of #1438, which was half-fixed by #1811 — it will no longer error as long as you don't type something in the field and delete it, which probably covers 99% of use. That fix is out on dogfood but not on colo.

[charliepark]

Should be fully fixed by #1854