DENG-17 Setting the start column on schedules to be nullable so that it (#23208)

Setting the start column on schedules to be nullable so that it can be dropped.

openedx/core/djangoapps/schedules/migrations/0015_schedules_start_nullable.py

+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.28 on 2020-03-04 21:49
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('schedules', '0014_historicalschedule_drop_fk'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='historicalschedule',
+            name='start',
+            field=models.DateTimeField(db_index=True, default=None, help_text='Date this schedule went into effect', null=True),
+        ),
+        migrations.AlterField(
+            model_name='schedule',
+            name='start',
+            field=models.DateTimeField(db_index=True, default=None, help_text='Date this schedule went into effect', null=True),
+        ),
+    ]

openedx/core/djangoapps/schedules/models.py

@@ -22,7 +22,9 @@ class Schedule(TimeStampedModel):
     # TODO Delete this field during last stage of rolling out field renames
     start = models.DateTimeField(
         db_index=True,
-        help_text=_('Date this schedule went into effect')
+        help_text=_('Date this schedule went into effect'),
+        null=True,
+        default=None
     )
     start_date = models.DateTimeField(
         db_index=True,