add upgrade script to remove non-null values from the sender column

migrations/versions/0085_govuk_sms_sender.py

@@ -0,0 +1,25 @@
+"""empty message
+
+Revision ID: 0085_govuk_sms_sender
+Revises: 0084_add_job_stats
+Create Date: 2017-05-22 13:46:09.584801
+
+"""
+
+# revision identifiers, used by Alembic.
+revision = '0085_govuk_sms_sender'
+down_revision = '0084_add_job_stats'
+
+from alembic import op
+
+
+def upgrade():
+    op.execute("UPDATE services SET sms_sender = 'GOVUK' where sms_sender is null")
+    op.execute("UPDATE services_history SET sms_sender = 'GOVUK' where sms_sender is null")
+    op.alter_column('services', 'sms_sender', nullable=False)
+    op.alter_column('services_history', 'sms_sender', nullable=False)
+
+
+def downgrade():
+    op.alter_column('services_history', 'sms_sender', nullable=True)
+    op.alter_column('services', 'sms_sender', nullable=True)