fix MergePollsVotes migration

plugins/poll/db/migrate/20151016163051_merge_polls_votes.rb

@@ -5,11 +5,11 @@ class MergePollsVotes < ActiveRecord::Migration
       polls_votes = {}
       PostCustomField.where(post_id: post_id).where("name LIKE 'polls-votes-%'").find_each do |pcf|
         user_id = pcf.name["polls-votes-".size..-1]
-        polls_votes["#{user_id}"] = ::JSON.parse(pcf.value)
+        polls_votes["#{user_id}"] = ::JSON.parse(pcf.value||"{}")
       end
 
       pcf = PostCustomField.find_or_create_by(name: "polls-votes", post_id: post_id)
-      pcf.value = ::JSON.parse(pcf.value).merge(polls_votes).to_json
+      pcf.value = ::JSON.parse(pcf.value||"{}").merge(polls_votes).to_json
       pcf.save
     end
   end