When attempting to load contents of a .blend, the code would just assume
if the number of added items is 0, that means it's not a .blend (but a
directory, although the previous commit fixed that part already).
However there may be situations where a .blend file simply doesn't
contain anything of interest to be added (e.g. when listing assets
only), so have a proper "none" value for this.
2910be8f19