clear the label if all messages have been removed, fixes #5346

Change-Id: I945a19f75ae7cb28e8acdd8489c5bbd11e831238
Reviewed-on: https://gerrit.instructure.com/5172
Tested-by: Hudson <hudson@instructure.com>
Reviewed-by: Zach Wily <zach@instructure.com>

app/models/conversation_participant.rb

@@ -198,6 +198,7 @@ class ConversationParticipant < ActiveRecord::Base
       self.last_message_at = nil
       self.has_attachments = false
       self.has_media_objects = false
+      self.label = nil
     end
     save
   end