Incorrectly asking to add users as contacts when in anonymous mode

When logged in in anonymous mode and with auto_join_private_chats set, ConverseJS asks "Would you like to add ...user... as a contact?"

I was using 12.0.0 but this doesn't occur in 11.0.1.

**To Reproduce**
Simply open a ConverseJS chat that's in anonymous login mode and have an automatically-joined private chat, as per above.

**Expected behavior**
Not to be asked to add anything as a contact.
