chore: Enable updating conversation additional attributes (#1611)

- Enable updating additional attributes via conversation API
- Handle the case when additional_attributes is nil

Fixes: #1357

app/controllers/api/v1/widget/messages_controller.rb

@@ -54,6 +54,8 @@ class Api::V1::Widget::MessagesController < Api::V1::Widget::BaseController
   end
 
   def conversation_params
+    # FIXME: typo referrer in additional attributes
+    # will probably require a migration.
     {
       account_id: inbox.account_id,
       inbox_id: inbox.id,