bug#52949: Traceback when reloading services

[Liliana Marie Prikler <liliana.prikler@gmail.com>]

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Hi Guix,

today I got the attached traceback while trying to reconfigure Guix. 
It turns out that 145:12 tries to check which services are currently
running and ignores that (eval exp) might return #f.

I'm not sure which point would be the correct one to guard against this
case, but a naïve approach would be to just check inside the (return
...) statement.  WDYT?

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guix system: Bootloader erfolgreich auf „(/boot/efi)“ installiert
Backtrace:
In guix/scripts/system.scm:
   1248:4 19 (_)
In ice-9/boot-9.scm:
  1752:10 18 (with-exception-handler _ _ #:unwind? _ # _)
In guix/store.scm:
   658:37 17 (thunk)
   1320:8 16 (call-with-build-handler _ _)
   1320:8 15 (call-with-build-handler _ _)
   1320:8 14 (call-with-build-handler _ _)
   1320:8 13 (call-with-build-handler _ _)
   1320:8 12 (call-with-build-handler #<procedure 7f015bf8f900 at g…> …)
  2123:24 11 (run-with-store #<store-connection 256.99 7f015bc599b0> …)
In ice-9/boot-9.scm:
  1747:15 10 (with-exception-handler #<procedure 7f0136028e40 at ic…> …)
In ice-9/exceptions.scm:
   406:15  9 (_)
In ice-9/boot-9.scm:
  1752:10  8 (with-exception-handler _ _ #:unwind? _ # _)
In guix/scripts/system.scm:
   865:15  7 (_)
In guix/store.scm:
  2123:24  6 (run-with-store #<store-connection 256.99 7f015b2ff960> …)
In guix/scripts/system.scm:
   865:15  5 (_ _)
In guix/scripts/system/reconfigure.scm:
   145:12  4 (_ _)
In srfi/srfi-1.scm:
    241:2  3 (map _ _)
In ice-9/boot-9.scm:
  1685:16  2 (raise-exception _ #:continuable? _)
  1683:16  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: #f