FIX: Skip migrating null/blank custom field values in migrations

db/migrate/20200728222920_create_discourse_voting_votes.rb

@@ -15,6 +15,7 @@ class CreateDiscourseVotingVotes < ActiveRecord::Migration[6.0]
       SELECT value::integer, user_id, 'false', created_at, updated_at
       FROM user_custom_fields
       WHERE name = 'votes'
+      AND value <> ''
       ON CONFLICT(user_id, topic_id) DO NOTHING
     SQL
 
@@ -23,6 +24,7 @@ class CreateDiscourseVotingVotes < ActiveRecord::Migration[6.0]
       SELECT value::integer, user_id, 'true', created_at, updated_at
       FROM user_custom_fields
       WHERE name = 'votes_archive'
+      AND value <> ''
       ON CONFLICT(user_id, topic_id) DO NOTHING
     SQL
   end

db/migrate/20200729042607_create_discourse_voting_topic_vote_count.rb

@@ -14,6 +14,7 @@ class CreateDiscourseVotingTopicVoteCount < ActiveRecord::Migration[6.0]
       SELECT topic_id::integer, value::integer, created_at, updated_at
       FROM topic_custom_fields
       WHERE name = 'vote_count'
+      AND value <> ''
     SQL
   end