monitoring: surround with mkMerge

modules/blocks/monitoring.nix

@@ -158,7 +158,8 @@ in
     };
   };
 
-  config = lib.mkIf cfg.enable {
+  config = lib.mkMerge [
+    (lib.mkIf cfg.enable {
       assertions = [
         {
           assertion = (!(isNull cfg.smtp)) -> builtins.length cfg.contactPoints > 0;
@@ -637,5 +638,6 @@ in
           };
         };
       };
-  };
+    })
+  ];
 }