Alerting: fix bug where user is able to access rules from namespaces user is not part of (#41403) (#41406)

* Add fix
* Add tests
(cherry picked from commit 6220872633)

Co-authored-by: Yuriy Tseretyan <yuriy.tseretyan@grafana.com>
Co-authored-by: Armand Grillet <2117580+armandgrillet@users.noreply.github.com>
Co-authored-by: Jean-Philippe Quéméner <JohnnyQQQQ@users.noreply.github.com>
Co-authored-by: George Robinson <george.robinson@grafana.com>
Co-authored-by: gotjosh <josue@grafana.com>

pkg/services/ngalert/api/api_ruler.go

@@ -152,6 +152,12 @@ func (srv RulerSrv) RouteGetRulesConfig(c *models.ReqContext) response.Response
 	if err != nil {
 		return ErrResp(http.StatusInternalServerError, err, "failed to get namespaces visible to the user")
 	}
+	result := apimodels.NamespaceConfigResponse{}
+
+	if len(namespaceMap) == 0 {
+		srv.log.Debug("User has no access to any namespaces")
+		return response.JSON(http.StatusOK, result)
+	}
 
 	namespaceUIDs := make([]string, len(namespaceMap))
 	for k := range namespaceMap {
@@ -215,7 +221,6 @@ func (srv RulerSrv) RouteGetRulesConfig(c *models.ReqContext) response.Response
 		}
 	}
 
-	result := apimodels.NamespaceConfigResponse{}
 	for namespace, m := range configs {
 		for _, ruleGroupConfig := range m {
 			result[namespace] = append(result[namespace], ruleGroupConfig)