Quickfix: lookup for first topic is expensive

since there are only 142 profiles that don't have one we will leave it
empty for now and don't show anything. In the next steps we will migrate
the data so every profile has a main topic again.

app/models/profile.rb

@@ -151,7 +151,9 @@ def name_or_email
   end
 
   def main_topic_or_first_topic
-    main_topic.present? ? main_topic : taggings.map { |tagging| tagging&.tag&.name }&.first
+    return nil unless main_topic
+
+    main_topic
   end
 
   # Try building a slug based on the following fields in