XF 2.1 How to force "dynamic letter avatar" for guest visitor?

[rdn]

I want to display custom avatars for logged members only.
And use "dynamic letter avatar" if guest browse the site.

On XF1, I can do it by editing:
library/XenForo/Template/Helper/Core.php line 1855

if (XenForo_Visitor::getInstance()->user_id && !empty($user['user_id']) && $forceType != 'default')

Thanks!