Workflow should return an error notification if a "Assign User to Workflow" action does not have a user or group assigned

[LukePercy]

Its possible to create a state in which a page is published and not able to be edited again if you do not assign a user or group to the action "Assign User to Workflow" action.

The workflow gets stuck in a state where pages publish and you cannot edit them anymore.

There should be a notification to ensure a User or group is set.