nixos/prometheus: get rid of empty arguments

Previously the prometheus.service file looked like:

  ExecStart=/nix/store/wjkhfw3xgkmavz1akkqir99w4lbqhak7-prometheus-1.8.2-bin/bin/prometheus -storage.local.path=/var/lib/prometheus/metrics \
    -config.file=/nix/store/zsnvzw51mk3n1cxjd0351bj39k1j6j27-prometheus.yml-check-config-checked \
    -web.listen-address=0.0.0.0:9090 \
    -alertmanager.notification-queue-capacity=10000 \
    -alertmanager.timeout=10s \
     \

  Restart=always

Now it's:

  ExecStart=/nix/store/wjkhfw3xgkmavz1akkqir99w4lbqhak7-prometheus-1.8.2-bin/bin/prometheus \
    -storage.local.path=/var/lib/prometheus/metrics \
    -config.file=/nix/store/zsnvzw51mk3n1cxjd0351bj39k1j6j27-prometheus.yml-check-config-checked \
    -web.listen-address=0.0.0.0:9090 \
    -alertmanager.notification-queue-capacity=10000 \
    -alertmanager.timeout=10s
  Restart=always