bug#45709: guix fails to print friendly help message for unknown command

[Imran Iqbal]

See below

~ >>> guix --version                                                            
                                                                              
guix (GNU Guix) e57b978fc0a6b6298b96c68de4769655d793c7b9
Copyright (C) 2021 the Guix authors
License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
~ >>> guix test                                                                 
                                                                              
Backtrace:
           6 (primitive-load "/home/imran/.config/guix/current/bin/g…")
In guix/ui.scm:
   2126:2  5 (run-guix-command test)
In ice-9/boot-9.scm:
  1731:15  4 (with-exception-handler #<procedure 7f74aeb360c0 at ic…> …)
In guix/ui.scm:
  2137:15  3 (_)
In ice-9/boot-9.scm:
   2806:4  2 (save-module-excursion #<procedure 7f74aeb4aa20 at ice-…>)
   4360:9  1 (_)
In unknown file:
           0 (string-prefix? "/" #f #<undefined> #<undefined> #<und…> …)

ERROR: In procedure string-prefix?:
In procedure string-prefix?: Wrong type argument in position 2 (expecting 
string): #f

It should be printing a helpful message saying "guix: test: command not found" 
followed by show-guix-usage but does not.