cups service: fix gutenprint update when there's no printers

(cherry picked from commit 851af5e8886dbedf3095006f9bfa5764db53b189)

nixos/modules/services/printing/cupsd.nix

···
               [ ! -e "/var/lib/cups/$i" ] && ln -s "${rootdir}/etc/cups/$i" "/var/lib/cups/$i"
             done
             ${optionalString cfg.gutenprint ''
-              ${gutenprint}/bin/cups-genppdupdate -p /etc/cups/ppd
+              if [ -d /var/lib/cups/ppd ]; then
+                ${gutenprint}/bin/cups-genppdupdate -p /var/lib/cups/ppd
+              fi
             ''}
           '';
       };