FIX: on topic page, don't try to render post counts for a deleted user

lib/topic_view.rb

@@ -212,6 +212,7 @@ class TopicView
 
   def post_counts_by_user
     @post_counts_by_user ||= Post.where(topic_id: @topic.id)
+                                 .where("user_id IS NOT NULL")
                                  .group(:user_id)
                                  .order("count_all DESC")
                                  .limit(24)

spec/components/topic_view_spec.rb

@@ -158,6 +158,12 @@ describe TopicView do
       it 'returns the two posters with their counts' do
         expect(topic_view.post_counts_by_user.to_a).to match_array([[first_poster.id, 2], [coding_horror.id, 1]])
       end
+
+      it "doesn't return counts for posts with authors who have been deleted" do
+        p2.user_id = nil
+        p2.save!
+        expect(topic_view.post_counts_by_user.to_a).to match_array([[first_poster.id, 2]])
+      end
     end
 
     context '.participants' do