Alerting: Fix notification channel migration and handle case when Alertmanager default configuration is absent (#35086)

* Fix dashboard alert and nootifier migration for MySQL

* Fix POSTing Alertmanager configuration if no current configuration exists

in case the default configuration has not be stored yet
or has failed to get stored

* Change CreatedAt field type

pkg/services/ngalert/api/api_alertmanager.go

@@ -192,11 +192,14 @@ func (srv AlertmanagerSrv) RoutePostAlertingConfig(c *models.ReqContext, body ap
 		}
 	}
 
-	currentConfig, err := notifier.Load([]byte(query.Result.AlertmanagerConfiguration))
-	if err != nil {
-		return ErrResp(http.StatusInternalServerError, err, "failed to load lastest configuration")
+	currentReceiverMap := make(map[string]*apimodels.PostableGrafanaReceiver)
+	if query.Result != nil {
+		currentConfig, err := notifier.Load([]byte(query.Result.AlertmanagerConfiguration))
+		if err != nil {
+			return ErrResp(http.StatusInternalServerError, err, "failed to load lastest configuration")
+		}
+		currentReceiverMap = currentConfig.GetGrafanaReceiverMap()
 	}
-	currentReceiverMap := currentConfig.GetGrafanaReceiverMap()
 
 	// Copy the previously known secure settings
 	for i, r := range body.AlertmanagerConfig.Receivers {

pkg/services/ngalert/models/alertmanager.go

@@ -1,7 +1,5 @@
 package models
 
-import "time"
-
 const AlertConfigurationVersion = 1
 
 // AlertConfiguration represents a single version of the Alerting Engine Configuration.
@@ -10,7 +8,7 @@ type AlertConfiguration struct {
 
 	AlertmanagerConfiguration string
 	ConfigurationVersion      string
-	CreatedAt                 time.Time `xorm:"created"`
+	CreatedAt                 int64 `xorm:"created"`
 	Default                   bool
 }