Adopt Rails 5 belongs_to_required_by_default

plugins/polls/app/models/poll_vote.rb

@@ -1,6 +1,6 @@
 class PollVote < ActiveRecord::Base
   belongs_to :poll
-  belongs_to :ordergroup
+  belongs_to :ordergroup, optional: true
   belongs_to :user
   has_many :poll_choices, dependent: :destroy
 end