Hello Guix,

This patch fixes the following error:
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$ guix import elpa -a melpa proc-net
Backtrace:
           6 (primitive-load "/home/natsu/.config/guix/current/bin/guix")
In guix/ui.scm:
  1570:12  5 (run-guix-command _ . _)
In guix/scripts/import.scm:
   114:11  4 (guix-import . _)
In guix/scripts/import/elpa.scm:
   108:23  3 (guix-import-elpa . _)
In guix/import/elpa.scm:
    247:2  2 (elpa->guix-package _ _)
   190:47  1 (fetch-elpa-package _ _)
In unknown file:
           0 (car ())

ERROR: In procedure car:
In procedure car: Wrong type (expecting pair): ()
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