install: losetup: check if the back-file attribute for a loop device is null (MR 2456)

Seems to be ChromeOS specific:

> this happens every time on my machine. I am on ChromeOS 129.0.6668.112
> using a Debian bookworm chroot through Crouton. I am not sure why
> this is happening nor which layer is responsible for this. I deleted
> the chroot and created another one but the problem persists.

Commit msg tweaked by Oliver.