Speaking as the (former) maintainer, this is really a bug in users-admin, but it's no longer developed since it's been replaced with the new GNOME control center applet upstream. (The new applet doesn't allow setting a password-less account.) If the bug had been reported against the gnome-system-tools, I'd had fixed the bug back in 2010, but now, it's too late for me... :-/
I don't think the workaround should be added in GDM, because I don't think it would be easy due to how PAM works.
Speaking as the (former) maintainer, this is really a bug in users-admin, but it's no longer developed since it's been replaced with the new GNOME control center applet upstream. (The new applet doesn't allow setting a password-less account.) If the bug had been reported against the gnome-system-tools, I'd had fixed the bug back in 2010, but now, it's too late for me... :-/
I don't think the workaround should be added in GDM, because I don't think it would be easy due to how PAM works.