Fix bug in cohorts controller that failed to update when a teacher email was missing

[aoby]

Error in the wild

ActiveRecord::AssociationTypeMismatch: User(#49552900) expected, got NilClass(#8696380)
  cohorts_controller.rb  132  update_teachers(...)

And sure enough, the unit test I just added failed with:

Minitest::UnexpectedError: ActiveRecord::AssociationTypeMismatch: User(#70110212537500) expected, got NilClass(#70110346146360)
    app/controllers/ops/cohorts_controller.rb:132:in `update_teachers'

The solution was straightforward. In update_teachers, the teachers param lists is mapped into User models representing the teachers, found or created on the fly, unless email is blank in which case that entry is nil. Then, later processing blew up on the nil User. Fix was simply compacting the user list to remove nils.