bug#50371: guix system init fail with "In procedure map: Wrong type argument"

["Sven Oliver Severini" <botfiddler@runbox.com>]

root@gnu /mnt/rootfs/boot# guix system init /mnt/rootfs/etc/config.scm /mnt/rootfs/
Backtrace:
In guix/ui.scm:
   2185:7 19 (run-guix . _)
  2148:10 18 (run-guix-command _ . _)
In ice-9/boot-9.scm:
  1752:10 17 (with-exception-handler _ _ #:unwind? _ # _)
In guix/status.scm:
    820:3 16 (_)
    800:4 15 (call-with-status-report _ _)
In guix/scripts/system.scm:
   1228:4 14 (_)
In ice-9/boot-9.scm:
  1752:10 13 (with-exception-handler _ _ #:unwind? _ # _)
In guix/store.scm:
   658:37 12 (thunk)
   1320:8 11 (call-with-build-handler #<procedure 7f5d1dcd15d0 at g\ufffd> \ufffd)
  2108:24 10 (run-with-store #<store-connection 256.99 7f5d1dcc1050> \ufffd)
In guix/scripts/system.scm:
  1252:15  9 (_ _)
    807:9  8 (perform-action init #<<image> name: #f format: disk-i\ufffd> \ufffd)
In gnu/system.scm:
  1327:26  7 (operating-system-bootcfg #<<operating-system> kernel:\ufffd> \ufffd)
   579:14  6 (operating-system-bootloader-crypto-devices #<<operatin\ufffd>)
In unknown file:
           5 (filter #<procedure 7f5d2c67bfa0 at gnu/system.scm:556\ufffd> \ufffd)
In gnu/system.scm:
   557:26  4 (_ _)
   533:17  3 (mapped-device-users #<<mapped-device> source: #<<uuid\ufffd> \ufffd)
In srfi/srfi-1.scm:
    241:2  2 (map _ _)
In ice-9/boot-9.scm:
  1685:16  1 (raise-exception _ #:continuable? _)
  1685:16  0 (raise-exception _ #:continuable? _)

ice-9/boot-9.scm:1685:16: In procedure raise-exception:
In procedure map: Wrong type argument: "rootfs"