UX: Don't display participants count if it is zero.

This can happen for megatopics where we don't send
down the payload for participants count.

app/assets/javascripts/discourse/widgets/topic-map.js.es6

@@ -125,12 +125,15 @@ createWidget("topic-map-summary", {
         h("h4", I18n.t("views_lowercase", { count: attrs.topicViews }))
       ])
     );
+
+    if (attrs.participantCount > 0) {
       contents.push(
         h("li.secondary", [
           numberNode(attrs.participantCount),
           h("h4", I18n.t("users_lowercase", { count: attrs.participantCount }))
         ])
       );
+    }
 
     if (attrs.topicLikeCount) {
       contents.push(