FIX: Do not move hidden post actions. (#7424)

Hidden (staff-only) post actions are whisper posts with no content, that
are later transformed by the client into post actions (discourse-assign
uses this).

app/models/post_mover.rb

@@ -224,6 +224,7 @@ class PostMover
     @posts ||= begin
       Post.where(topic: @original_topic, id: post_ids)
         .where.not(post_type: Post.types[:small_action])
+        .where.not(raw: '')
         .order(:created_at).tap do |posts|
 
         raise Discourse::InvalidParameters.new(:post_ids) if posts.empty?