fix null check in constraint

There are two fun quirks of postgres/sql that we need to work around:
* any `x = y` where x or y is NULL returns NULL, rather than false.
* check constraints accept NULL or true values as good.

so, the check `postage in ('first', 'second')` returns `null` rather
than `false` when postage is null itself. This surprisingly passes the
check constraint. To get around this, we have to add an explicit not
null check as well.

migrations/versions/0230_noti_postage_constraint.py

@@ -17,7 +17,7 @@ def upgrade():
         ALTER TABLE notifications ADD CONSTRAINT "chk_notifications_postage_null"
         CHECK (
             CASE WHEN notification_type = 'letter' THEN
-                postage in ('first', 'second')
+                postage is not null and postage in ('first', 'second')
             ELSE
                 postage is null
             END
@@ -28,7 +28,7 @@ def upgrade():
         ALTER TABLE notification_history ADD CONSTRAINT "chk_notification_history_postage_null"
         CHECK (
             CASE WHEN notification_type = 'letter' THEN
-                postage in ('first', 'second')
+                postage is not null and postage in ('first', 'second')
             ELSE
                 postage is null
             END