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/accounts/conversations_controller.rb

@@ -86,7 +86,8 @@ class Api::V1::Accounts::ConversationsController < Api::V1::Accounts::BaseContro
       account_id: Current.account.id,
       inbox_id: @contact_inbox.inbox_id,
       contact_id: @contact_inbox.contact_id,
-      contact_inbox_id: @contact_inbox.id
+      contact_inbox_id: @contact_inbox.id,
+      additional_attributes: params[:additional_attributes]
     }
   end