role field cannot be null when changing ip address via API

[orgito]

Environment

• Python version: 3.4.8
• NetBox version: 2.4.0

Steps to Reproduce

Change an IP address with the role field as null.

Expected Behavior

Should save the IP address

Observed Behavior

Fails with the errror:

{"role":["This field may not be null."]}

Adding allow_null=True to https://github.com/digitalocean/netbox/blob/f59682a7c955baa6b336ebe927d69c26f725ad67/netbox/ipam/api/serializers.py#L261 solves the issue

[jeremystretch]

I've found this same issue in several places where ChoiceField is used on a nullable field.