Empty column and false unread counts with private replies

When sending private replies, there are instances where the Last Post will be blank.

Steps to replicate:

1. As an admin, create a forum and a discussion.
2. Go to the discussion created
3. Reply privately to the newly created discussion by selecting the 'Private Reply' checkbox and submit the form.
4. As a student go to the forum
5. Notice the columns Unread with value "1" and Last Post which is empty. e.g.