bug#28987: Another fail report for `guix import crate`.

[Fis Trivial]

Version :5ae8c77adec6b64a53ed10c3505263380205321d

$ guix import crate racer

--8<---------------cut here---------------start------------->8---
Backtrace:
           5 (primitive-load "/gnu/store/nxdxbad50ydp1nhmgzvlwbk0jkk…")
In guix/ui.scm:
  1474:12  4 (run-guix-command _ . _)
In guix/scripts/import.scm:
   114:11  3 (guix-import . _)
In guix/scripts/import/crate.scm:
    86:19  2 (guix-import-crate . _)
In guix/import/crate.scm:
    49:30  1 (crate-fetch _ #<procedure make-crate-sexp (#:key name …>)
In unknown file:
           0 (string-split #f #\/)

ERROR: In procedure string-split:
ERROR: In procedure string-split: Wrong type argument in position 1 (expecting 
string): #f
-8<---------------cut here---------------end--------------->8---

It seems not many people would try to use guix with rust. :(
If I could get geiser working in next few weeks, I might be able to help 
finding some clues.