Alertmanager notification service doesn't work

Using notification controller bundled in argocd:v2.4.8 with config:
```
service.alertmanager: |
targets:
- webhook.site:443
scheme: https
apiPath: /0be1114e-c211-4fb3-bb47-908e51aa9675/api/events
```

and app annotation
```
notifications.argoproj.io/subscribe.on-sync-succeeded.alertmanager: ""
```

I can't get any notifications on my endpoint. I can see this error in logs:
```
time="2023-01-06T20:59:45Z" level=error msg="Failed to notify recipient {alertmanager } defined in resource argocd/guestbook: notification alertmanager no config" resource=argocd/guestbook
```

Start by reproducing the issue with the Alertmanager service configuration and the on-sync-succeeded subscription annotation shown in the report, then inspect the notification controller logs for the “notification alertmanager no config” error. Done means the configured webhook receives the notification and the no-config error no longer appears.

Written by the indexing model from the issue text.