nixos/mosquitto: fix "listeners" option default

The option type is listOf, so the default should be an empty list, not
empty attrset.

Author: bjornfor

nixos/modules/services/networking/mosquitto.nix

@@ -483,7 +483,7 @@ let
 
     listeners = mkOption {
       type = listOf listenerOptions;
-      default = {};
+      default = [];
       description = ''
         Listeners to configure on this broker.
       '';