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?