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 :-)

1 files changed, 2 insertions, 2 deletions

diff --git a/nixos/modules/services/monitoring/prometheus/alertmanager.nix b/nixos/modules/services/monitoring/prometheus/alertmanager.nix
index a9c0ce4ed6cb..da2cd02eaa3b 100644
--- a/nixos/modules/services/monitoring/prometheus/alertmanager.nix
+++ b/nixos/modules/services/monitoring/prometheus/alertmanager.nix
@@ -62,8 +62,8 @@ in {
       };
 
       listenAddress = mkOption {
-        type = types.nullOr types.str;
-        default = null;
+        type = types.str;
+        default = "";
         description = ''
           Address to listen on for the web interface and API.
         '';