Do not move CWs to toot body when toot body is empty

Fixes #395

Instead of leaving the toot body blank, it replaces it with a single “.” in
order for the fold/unfold CW behavior to not look *too* weird on upstream
Mastodon. Note that this does not fix upstream's CW-dropping behavior, as
that is decided at the time the toot is posted, not received.

app/services/post_status_service.rb

@@ -21,7 +21,7 @@ class PostStatusService < BaseService
 
     media  = validate_media!(options[:media_ids])
     status = nil
-    text   = options.delete(:spoiler_text) if text.blank? && options[:spoiler_text].present?
+    text   = '.' if text.blank? && options[:spoiler_text].present?
 
     ApplicationRecord.transaction do
       status = account.statuses.create!(text: text,