FIX: Don't allow NULL values for notification_level in category_users (#15407)

db/migrate/20211224111749_not_null_notification_level_in_category_users.rb

@@ -0,0 +1,10 @@
+# frozen_string_literal: true
+
+class NotNullNotificationLevelInCategoryUsers < ActiveRecord::Migration[6.1]
+  def change
+    up_only do
+      execute("DELETE FROM category_users WHERE notification_level IS NULL")
+    end
+    change_column_null :category_users, :notification_level, false
+  end
+end