It doesn't matter, search engines can't see your acp so anything to do with search visibility, ranking, accessibility etc doesn't really benefit in any way here.
When you are not logged in there is alt text -
<img src="styles/default/xenforo/XenForo.png" id="loginLogo" alt="XenForo Logo">
and when you are logged in there is not which again dsoesn't matter because google is never going to see the logged in view of your ACP.
Alt text is a good habit but I don't know it it benefits to change anything here...unless we are talking about different logos here and what I said doesn't apply to it.