FIX: Some solved topics have no answers (#350)

As part of https://github.com/discourse/discourse-solved/pull/342, some discrepancies in the old implementation resulted in solved topics linking to posts that do not exist (dangling custom field value). Causing us to see the following when there are no `answer_post`

```
NoMethodError (undefined method `user' for nil)
```

This PR prevents the error.

lib/discourse_solved/topic_view_serializer_extension.rb

@@ -6,7 +6,8 @@ module DiscourseSolved::TopicViewSerializerExtension
   prepended { attributes :accepted_answer }
 
   def include_accepted_answer?
-    SiteSetting.solved_enabled? && object.topic.solved.present?
+    SiteSetting.solved_enabled? && object.topic.solved.present? &&
+      object.topic.solved.answer_post.present?
   end
 
   def accepted_answer

spec/serializers/topic_view_serializer_spec.rb

@@ -0,0 +1,35 @@
+# frozen_string_literal: true
+
+describe TopicViewSerializer do
+  fab!(:topic)
+  fab!(:post1) { Fabricate(:post, topic:) }
+  fab!(:post2) { Fabricate(:post, topic:) }
+  fab!(:user)
+
+  before { SiteSetting.solved_enabled = true }
+
+  describe "#accepted_answer" do
+    it "returns the accepted answer post when the topic has an accepted answer" do
+      Fabricate(:solved_topic, topic: topic, answer_post: post2)
+      serializer = TopicViewSerializer.new(TopicView.new(topic), scope: Guardian.new(user))
+      serialized = serializer.as_json
+      expect(serialized[:topic_view][:accepted_answer][:post_number]).to eq(post2.post_number)
+    end
+
+    it "returns nil when the topic does not have an accepted answer" do
+      unsolved_topic = Fabricate(:topic)
+      serializer = TopicViewSerializer.new(TopicView.new(unsolved_topic), scope: Guardian.new(user))
+      serialized = serializer.as_json
+      expect(serialized[:accepted_answer]).to be_nil
+    end
+
+    it "returns nil when the accepted answer post does not exist" do
+      weird_topic = Fabricate(:solved_topic)
+      weird_topic.update!(answer_post_id: 19_238_319)
+      serializer =
+        TopicViewSerializer.new(TopicView.new(weird_topic.topic), scope: Guardian.new(user))
+      serialized = serializer.as_json
+      expect(serialized[:accepted_answer]).to be_nil
+    end
+  end
+end