`AutocompleteController#users` should not break when `current_user` is `nil`
https://sentry.gitlap.com/gitlab/gitlabcom/issues/15375/
NoMethodError: undefined method `id' for nil:NilClass
app/controllers/autocomplete_controller.rb:15:in `users'
@users = @users.todo_authors(current_user.id, params[:todo_state_filter])
lib/gitlab/request_profiler/middleware.rb:15:in `call'
@app.call(env)
lib/gitlab/middleware/rails_queue_duration.rb:20:in `call'
@app.call(env)
lib/gitlab/metrics/rack_middleware.rb:29:in `block in call'
retval = trans.run { @app.call(env) }
lib/gitlab/metrics/transaction.rb:49:in `run'
yield
...
(93 additional frame(s) were not displayed)
AutocompleteController#users
doesn't require an authenticated user (so that unauthenticated are able to filter by user) so we should handle current_user
being nil
gracefully in this action.