nixos/atop: Don't choke if no existing atop logs

Changed files
+1
nixos
modules
programs
+1
nixos/modules/programs/atop.nix
···
# convert remainings logs and start eventually
atop.serviceConfig.ExecStartPre = pkgs.writeShellScript "atop-update-log-format" ''
set -e -u
+
shopt -s nullglob
for logfile in "$LOGPATH"/atop_*
do
${atop}/bin/atopconvert "$logfile" "$logfile".new