I thought that one of the important things is for people with visual impairment:
Alternative text is text associated with an image that serves the same purpose and conveys the same essential information as the image. In situations where the image is not available to the reader, perhaps because they have turned off images in their web browser or are using a screen reader due to a visual impairment, the alternative text ensures that no information or functionality is lost.
In that case we should use alt text to be explicit IMO.
That's what shows on your acp now when at the login screen AKA what the public would see.
I don't see much functionality being lost by not displaying alt text in place of a logo that only the site owners for the most part will ever see anyways, I get your point, but the logo does not do much in that place and personally I have never even thought about clicking it in the ACP to get the public homepage until now because the same link is essentially right below it to the right in the form of your site name as a link right before the search so no functionality is actually lost imo as I have always thought that the logo was there as a nice touch and as a link for people who didn't belong at that ACP login page to get back tot he frontend easily, once logged in the logo doesn't serve much purpose to a site owner..
I'm not against following common practices I just don't see this as an accessibility issue since to the public...it has alt text (what I posted above is currently what it is at ACP login) and for the most part people should be accessing that page anyways and if they shouldn't be, they will almost certainly not see the logged in view of that image which has no alt text