Fix preload_first_topic_post on empty association

Reported in f1874fb#r33825204

app/models/concerns/thredded/post_common.rb

@@ -22,6 +22,7 @@ module PostCommon
         posts_table_name = quoted_table_name
         result = all
         owners_by_id = result.each_with_object({}) { |r, h| h[r.postable_id] = r.postable }
+        next result if owners_by_id.empty?
         preloader = ActiveRecord::Associations::Preloader.new.preload(
           owners_by_id.values, :first_post,
           unscoped.where(<<~SQL.delete("\n"))

app/models/thredded/post_moderation_record.rb

@@ -31,6 +31,7 @@ class PostModerationRecord < ActiveRecord::Base
       posts_table_name = Thredded::Post.quoted_table_name
       result = all
       owners_by_id = result.each_with_object({}) { |r, h| h[r.post.postable_id] = r.post.postable }
+      next result if owners_by_id.empty?
       preloader = ActiveRecord::Associations::Preloader.new.preload(
         owners_by_id.values, :first_post,
         Thredded::Post.unscoped.where(<<~SQL.delete("\n"))

spec/models/thredded/post_moderation_record_spec.rb

@@ -0,0 +1,9 @@
+# frozen_string_literal: true
+
+require 'spec_helper'
+
+RSpec.describe Thredded::PostModerationRecord do
+  it 'empty preload_first_topic_post' do
+    expect { described_class.preload_first_topic_post.to_a }.not_to raise_error
+  end
+end