Change current_user so it actually returns nil

User.find(session[:user_id]) doesn't actually return nil, it throws the following error: 
Failure/Error: expect(controller.current_user.id).to eq(user.id)
     ActiveRecord::RecordNotFound:
       Couldn't find User without an ID

If we actually want nil (which we probably do if we want a falsey value for if statements later) we can do User.where(id: session[:user_id]).take.