Make service_broadcast_settings.provider non-nullable

We set all existing null values to "all", then make the column
non-nullable. Admin is already passing through the value of "all".

migrations/versions/0353_broadcast_provider_not_null.py

@@ -0,0 +1,22 @@
+"""
+
+Revision ID: 0353_broadcast_provider_not_null
+Revises: 0352_broadcast_provider_types
+Create Date: 2021-05-10 15:06:40.046786
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+revision = '0353_broadcast_provider_not_null'
+down_revision = '0352_broadcast_provider_types'
+
+
+def upgrade():
+    op.execute("UPDATE service_broadcast_settings SET provider = 'all' WHERE provider is null")
+    op.alter_column('service_broadcast_settings', 'provider', existing_type=sa.VARCHAR(), nullable=False)
+
+
+def downgrade():
+    op.alter_column('service_broadcast_settings', 'provider', existing_type=sa.VARCHAR(), nullable=True)
+    op.execute("UPDATE service_broadcast_settings SET provider = null WHERE provider = 'all'")