Set Program.excluded_course_runs to be an optional field (#241)

ECOM-5097

course_discovery/apps/course_metadata/migrations/0015_auto_20160813_2142.py

+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('course_metadata', '0014_auto_20160811_0436'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='program',
+            name='excluded_course_runs',
+            field=models.ManyToManyField(blank=True, null=True, to='course_metadata.CourseRun'),
+        ),
+    ]

course_discovery/apps/course_metadata/models.py

@@ -556,7 +556,7 @@ class Program(TimeStampedModel):
     courses = models.ManyToManyField(Course)
     # NOTE (CCB): Editors of this field should validate the values to ensure only CourseRuns associated
     # with related Courses are stored.
-    excluded_course_runs = models.ManyToManyField(CourseRun)
+    excluded_course_runs = models.ManyToManyField(CourseRun, blank=True, null=True)
     partner = models.ForeignKey(Partner, null=True, blank=False)
     overview = models.TextField(null=True, blank=True)
     weeks_to_complete = models.PositiveSmallIntegerField(null=True, blank=True)