fix migration

apps/news/migrations/0014_auto_20190927_0845.py

@@ -1,11 +1,12 @@
 # Generated by Django 2.2.4 on 2019-09-27 08:45
 from django.db import migrations
+from django.core.validators import EMPTY_VALUES
 
 
 def fill_slug(apps,schemaeditor):
     News = apps.get_model('news', 'News')
     for news in News.objects.all():
-        if news.slug is None:
+        if news.slug in EMPTY_VALUES:
             news.slug = f'Slug_{news.id}'
             news.save()