Allow Departments to NOT have an admin assistant

[kevinreiss]

User story

As an adminstrator a department may not have admin assistant at times. I should be able to request that a department have no one assigned this role.

Concrete example

Assistant X leaves their department, the role is vacant until it is filled.

Acceptance criteria

• A department can have no active admin assistants defined in config/departments.yml
• The daily staff report still can be run successfully.