Fix comments_since_last_visit on first visit

On a user's first visit to a topic, this attribute wasn't being set.

tildes/tildes/models/topic/topic_query.py

@@ -109,6 +109,8 @@ class TopicQuery(PaginatedQuery):
             topic.bookmark_created_time = result.created_time
 
             topic.last_visit_time = result.visit_time
+
+            topic.comments_since_last_visit = None
             if result.num_comments is not None:
                 new_comments = topic.num_comments - result.num_comments
                 # prevent showing negative "new comments" due to deletions

tildes/tildes/templates/topic.jinja2

@@ -177,7 +177,7 @@
         {% if request.user
              and request.user.track_comment_visits
              and not request.user.collapse_old_comments
-             and topic.comments_since_last_visit > 0 %}
+             and topic.comments_since_last_visit %}
           <button class="btn btn-sm btn-light" data-js-comment-collapse-read-button>Collapse read</button>
         {% endif %}