fix default empty groups for k8s PrometheusRule

jsonnet/kube-prometheus/components/k8s-control-plane.libsonnet

@@ -47,8 +47,8 @@ function(params) {
       labels+: k8s._config.mixin.ruleLabels,
     },
     spec: {
-      local r = if std.objectHasAll(k8s.mixin, 'prometheusRules') then k8s.mixin.prometheusRules.groups else {},
-      local a = if std.objectHasAll(k8s.mixin, 'prometheusAlerts') then k8s.mixin.prometheusAlerts.groups else {},
+      local r = if std.objectHasAll(k8s.mixin, 'prometheusRules') then k8s.mixin.prometheusRules.groups else [],
+      local a = if std.objectHasAll(k8s.mixin, 'prometheusAlerts') then k8s.mixin.prometheusAlerts.groups else [],
       groups: a + r,
     },
   },