Test that alert with no references is OK
Test that alert with no references is OK

References are optional, and we fixed errors when they are not provided
in bbc444699a6032ef855959d81aee905775be9862

This commit amends the tests so that they cover the `Alert` message type
as well as the `Cancel` (which is already covered).

alphagov/notifications-api,alphagov/notifications-api