nixos/grafana: fix evaluation when no settings is defined (#354361)

Author: SuperSandro2000

nixos/modules/services/monitoring/grafana.nix

@@ -255,6 +255,7 @@ in
         Grafana settings. See <https://grafana.com/docs/grafana/latest/setup-grafana/configure-grafana/>
         for available options. INI format is used.
       '';
+      default = { };
       type = types.submodule {
         freeformType = settingsFormatIni.type;