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.

app/models.py

@@ -1193,7 +1193,7 @@ class Notification(db.Model):
     postage = db.Column(db.String, nullable=True)
     CheckConstraint("""
         CASE WHEN notification_type = 'letter' THEN
-            postage in ('first', 'second')
+            postage is not null and postage in ('first', 'second')
         ELSE
             postage is null
         END
@@ -1453,7 +1453,7 @@ class NotificationHistory(db.Model, HistoryModel):
     postage = db.Column(db.String, nullable=True)
     CheckConstraint("""
         CASE WHEN notification_type = 'letter' THEN
-            postage in ('first', 'second')
+            postage is not null and postage in ('first', 'second')
         ELSE
             postage is null
         END