FEATURE: Fallback to system users when creating new TopicEmbed (#12386)

Previosly, if the topic embed request had a missing username parameter,
and SiteSetting.embed_by_username was empty we would fail to create the
new topic and not show any errors.

Now we will fallback using the priority:

1. Username parameter
2. SiteSetting.embed_by_username
3. SiteSetting.site_contact_username
4. system user

lib/topic_retriever.rb

@@ -41,7 +41,7 @@ class TopicRetriever
 
   def fetch_http
     if @author_username.nil?
-      username = SiteSetting.embed_by_username.downcase
+      username = SiteSetting.embed_by_username.presence || SiteSetting.site_contact_username.presence || Discourse.system_user.username
     else
       username = @author_username
     end