.Site.Home gives the same result. I’d expect it to be empty, but it is the value of title instead. I guess I can work around it by comparing if the match or not.
I can check if an index file (not) exists with {{ (in .Site.Home .Site.Title ) }}, but I thought I could do something like {{ (eq .Site.Home "") }}.
I also found out, that an another way to check, if an index file is present is {{ (eq .Title "" ) }}, as (unlike .Site.Title), .Title is only present, if there is no index file.