Cable API endpoint refuses a null length_unit

[aruhier]

Environment

• Python version: 3.6.7
• NetBox version: 2.5.7

Steps to Reproduce

1. Get a cable with no length/length_unit (None for both) through the API
2. PUT the exact cable data you received

What should happen: Nothing, the cable should be updated with the same data it had before.

What happens: 400 error, {'length_unit': ['This field may not be null.']}