Allow null & blank for PickleField. (#315)

The default behaviour of PickleField was changed from null=True to
null=False. This causes issues with fields such as an image_field which
will try storing a NULL when unset.

constance/backends/database/migrations/0002_auto_20190129_2304.py

@@ -0,0 +1,19 @@
+# Generated by Django 2.1.5 on 2019-01-30 04:04
+
+from django.db import migrations
+import picklefield.fields
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('database', '0001_initial'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='constance',
+            name='value',
+            field=picklefield.fields.PickledObjectField(blank=True, editable=False, null=True),
+        ),
+    ]

constance/backends/database/models.py

@@ -13,7 +13,7 @@ except ImportError:
 
 class Constance(models.Model):
     key = models.CharField(max_length=255, unique=True)
-    value = PickledObjectField()
+    value = PickledObjectField(null=True, blank=True)
 
     class Meta:
         verbose_name = _('constance')