Filtering on null values for nullable CharFields is broken

[jeremystretch]

Environment

• Python version: 3.5.2
• NetBox version: 2.5.6

Steps to Reproduce

1. Attempt to filter the racks list by null facility_id value:

http://netbox/dcim/racks/?facility_id=null

Expected Behavior

Only racks with a null facility_id should be returned.

Observed Behavior

The filter has no effect. There are two issues at play here.

First, NullableCharFieldFilter is looking for a query parameter of NULL rather than null as specified in settings.py. Second, if NULL is passed, an exception occurs due to the filter using obsolete syntax: self.name should be self.field_name.