Alerting: Assume built-in AM is receiving alerts in case of not having admin config (#87893)

public/app/features/alerting/unified/api/alertmanagerApi.ts

@@ -94,7 +94,7 @@ export const alertmanagerApi = alertingApi.injectEndpoints({
 
     // this endpoint requires administrator privileges
     getGrafanaAlertingConfiguration: build.query<GrafanaAlertingConfiguration, void>({
-      query: () => ({ url: '/api/v1/ngalert/admin_config' }),
+      query: () => ({ url: '/api/v1/ngalert/admin_config', showErrorAlert: false }),
       providesTags: ['AlertingConfiguration'],
     }),
 

public/app/features/alerting/unified/utils/settings.ts

@@ -2,7 +2,12 @@ import { AlertmanagerChoice, GrafanaAlertingConfiguration } from 'app/plugins/da
 
 // if we have either "internal" or "both" configured this means the internal Alertmanager is receiving Grafana-managed alerts
 export const isInternalAlertmanagerInterestedInAlerts = (config?: GrafanaAlertingConfiguration): boolean => {
-  switch (config?.alertmanagersChoice) {
+  if (!config) {
+    // The backend doesn't have a configuration record in a new Grafana instance until the user has interacted with the configuration page.
+    // For that reason, in case of no configuration, we assume that the internal Alertmanager is interested in alerts.
+    return true;
+  }
+  switch (config.alertmanagersChoice) {
     case AlertmanagerChoice.Internal:
     case AlertmanagerChoice.All:
       return true;