FIX: summary email excludes all untagged topics if user has muted some tags

app/models/topic.rb

@@ -373,7 +373,7 @@ class Topic < ActiveRecord::Base
     muted_tag_ids = TagUser.lookup(user, :muted).pluck(:tag_id)
     unless muted_tag_ids.empty?
       topics = topics.joins("LEFT OUTER JOIN topic_tags ON topic_tags.topic_id = topics.id")
-                     .where("topic_tags.tag_id NOT IN (?)", muted_tag_ids)
+                     .where("topic_tags.tag_id NOT IN (?) OR topic_tags.tag_id is null", muted_tag_ids)
     end
 
     topics

spec/models/topic_spec.rb

@@ -1413,6 +1413,17 @@ describe Topic do
       expect(Topic.for_digest(user, 1.year.ago, top_order: true)).to eq([topic])
     end
 
+    it "returns topics with no tags too" do
+      user = Fabricate(:user)
+      muted_tag, other_tag = Fabricate(:tag), Fabricate(:tag)
+      TagUser.change(user.id, muted_tag.id, TagUser.notification_levels[:muted])
+      topic1 = Fabricate(:topic, tags: [muted_tag])
+      topic2 = Fabricate(:topic, tags: [other_tag])
+      topic3 = Fabricate(:topic)
+
+      expect(Topic.for_digest(user, 1.year.ago, top_order: true)).to contain_exactly(topic2, topic3)
+    end
+
     it "sorts by category notification levels" do
       category1, category2 = Fabricate(:category), Fabricate(:category)
       2.times {|i| Fabricate(:topic, category: category1) }