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.

pmb/install/losetup.py

@@ -72,7 +72,7 @@ def device_by_back_file(back_file: Path) -> Path:
     # Find the back_file
     losetup = json.loads(losetup_output)
     for loopdevice in losetup["loopdevices"]:
-        if Path(loopdevice["back-file"]) == back_file:
+        if loopdevice["back-file"] is not None and Path(loopdevice["back-file"]) == back_file:
             return Path(loopdevice["name"])
     raise RuntimeError(f"Failed to find loop device for {back_file}")