netbox: Non-ascii tag creates empty slug, exception django.urls.exceptions.NoReverseMatch

Environment

• Python version: 3.5.2
• NetBox version: 2.6.7

Steps to Reproduce

1. edit a device
2. add a tag consisting soley of non-ASCII characters, e.g. 台灣
3. save
4. go to Organization > Tags (/extras/tags/) in the GUI

Expected Behavior

Tag to be functional

Observed Behavior

I get an exception:

<class 'django.urls.exceptions.NoReverseMatch'>

Reverse for 'tag' with arguments '('',)' not found. 1 pattern(s) tried: ['extras\\/tags\\/(?P<slug>[-a-zA-Z0-9_]+)\\/$']

If I query the database, I find that a tag has been created with an empty slug (hence cannot match the regexp pattern which requires at least one character)

netbox=# select * from extras_tag;
 id | name | slug | color  | comments |  created   |         last_updated
----+------+------+--------+----------+------------+-------------------------------
  1 | temp | temp | 9e9e9e |          | 2019-10-12 | 2019-10-12 16:56:36.745793+00
  2 | 台灣 |      | 9e9e9e |          | 2019-11-18 | 2019-11-18 08:07:13.025115+00
(2 rows)

I also find I get this exception at the home page (/), but not at /dcim/devices/.

Cleanup

netbox=# delete from extras_taggeditem where tag_id in (select id from extras_tag where slug='');
DELETE 1
netbox=# delete from extras_tag where slug='';
DELETE 1

The exception persists for 15 minutes or until you invalidate the cacheops cache:

cd /opt/netbox/netbox
python3 manage.py nbshell
>>> import cacheops
>>> cacheops.invalidate_all()

Links

Reported previously as #2853, #3079, #3717 (insufficient detail to reproduce).

Reported on the google group here, here, here, here.