set color='' to color=None

src/c3nav/mapdata/migrations/0015_auto_20170706_1334.py

@@ -0,0 +1,22 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.2 on 2017-07-06 11:34
+from __future__ import unicode_literals
+
+from django.db import migrations
+
+
+def set_blank_to_null(apps, schema_editor):
+    for model_name in ('Area', 'Point', 'Level', 'Space', 'LocationGroup'):
+        model = apps.get_model('mapdata', model_name)
+        model.objects.filter(color='').update(color=None)
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('mapdata', '0014_mapupdate'),
+    ]
+
+    operations = [
+        migrations.RunPython(set_blank_to_null),
+    ]