remove useless null=True on many2many fields

nonstop/migrations/0004_auto_20170418_1525.py

+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('nonstop', '0003_track_nonstop_zones'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='track',
+            name='nonstop_zones',
+            field=models.ManyToManyField(to='emissions.Nonstop', blank=True),
+        ),
+    ]

nonstop/models.py

@@ -38,7 +38,7 @@ class Track(models.Model):
             choices=LANGUAGES, blank=True)
     sabam = models.BooleanField('SABAM', default=True)
     cfwb = models.BooleanField('CFWB', default=False)
-    nonstop_zones = models.ManyToManyField('emissions.Nonstop', null=True, blank=True)
+    nonstop_zones = models.ManyToManyField('emissions.Nonstop', blank=True)
 
     def get_absolute_url(self):
         return reverse('track-view', kwargs={'pk': self.id})