nixos/octoprint: fix ipv6 host (#351721)

nixos/modules/services/misc/octoprint.nix

···
 
   baseConfig = {
     plugins.curalegacy.cura_engine = "${pkgs.curaengine_stable}/bin/CuraEngine";
-    server.host = cfg.host;
     server.port = cfg.port;
     webcam.ffmpeg = "${pkgs.ffmpeg.bin}/bin/ffmpeg";
-  };
+  } // lib.optionalAttrs (cfg.host != null) {server.host = cfg.host;};
 
   fullConfig = lib.recursiveUpdate cfg.extraConfig baseConfig;
 
···
       enable = lib.mkEnableOption "OctoPrint, web interface for 3D printers";
 
       host = lib.mkOption {
-        type = lib.types.str;
-        default = "0.0.0.0";
+        type = lib.types.nullOr lib.types.str;
+        default = null;
         description = ''
           Host to bind OctoPrint to.
         '';