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 :-)

nixos/modules/services/monitoring/prometheus/alertmanager.nix

···
       };
 
       listenAddress = mkOption {
-        type = types.nullOr types.str;
-        default = null;
+        type = types.str;
+        default = "";
         description = ''
           Address to listen on for the web interface and API.
         '';