nixos/prometheus: unbreak alertmanager default config

The current default value of listenAddress = null blows up:

  $ nixos-rebuild build
  error: cannot coerce null to a string, at
  .../nixpkgs/nixos/modules/services/monitoring/prometheus/alertmanager.nix:97:16

With listenAddress = "" we use the same default as upstream and there is
no blow up :-)