FIX: Sometimes `contents` is nil, don't break in that case.

app/models/topic_embed.rb

@@ -20,6 +20,7 @@ class TopicEmbed < ActiveRecord::Base
     if SiteSetting.embed_truncate
       contents = first_paragraph_from(contents)
     end
+    contents ||= ''
     contents << imported_from_html(url)
 
     url = normalize_url(url)