Allow None FK on Submission model

Turns out you have to let the database know that a value can be None,
who knew? Migration included.

submissions/migrations/0003_submission_null_FK.py

+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('submissions', '0002_auto_20151119_0913'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='submission',
+            name='student_item',
+            field=models.ForeignKey(to='submissions.StudentItem', null=True),
+        ),
+    ]

submissions/models.py

@@ -103,7 +103,7 @@ class Submission(models.Model):
 
     uuid = UUIDField(version=1, db_index=True)
 
-    student_item = models.ForeignKey(StudentItem)
+    student_item = models.ForeignKey(StudentItem, null=True)
 
     # Which attempt is this? Consecutive Submissions do not necessarily have
     # increasing attempt_number entries -- e.g. re-scoring a buggy problem.