Serialize text-less statuses as '.' over OStatus (fixes #7856) (#8126)

app/lib/ostatus/atom_serializer.rb

@@ -352,7 +352,7 @@ class OStatus::AtomSerializer
     append_element(entry, 'link', nil, rel: :alternate, type: 'application/activity+json', href: ActivityPub::TagManager.instance.uri_for(status)) if status.account.local?
 
     append_element(entry, 'summary', status.spoiler_text, 'xml:lang': status.language) if status.spoiler_text?
-    append_element(entry, 'content', Formatter.instance.format(status).to_str, type: 'html', 'xml:lang': status.language)
+    append_element(entry, 'content', Formatter.instance.format(status).to_str || '.', type: 'html', 'xml:lang': status.language)
 
     status.mentions.sort_by(&:id).each do |mentioned|
       append_element(entry, 'link', nil, rel: :mentioned, 'ostatus:object-type': OStatus::TagManager::TYPES[:person], href: OStatus::TagManager.instance.uri_for(mentioned.account))