(fleet) fix telemetry default site when no site is set (#43334)

This PR fixes the default site used by the installer daemon telemetry when no `site` is set in the config.

We had a report of the following error:
```
2025-11-20 14:33:52 EST | INSTALLER | WARN | (pkg/fleet/installer/telemetry/client.go:249 in func1) | failed to send telemetry payload to endpoint https://instrumentation-telemetry-intake./api/v2/apmtelemetry: Post "https://instrumentation-telemetry-intake./api/v2/apmtelemetry": dial tcp: lookup instrumentation-telemetry-intake.: Temporary failure in name resolution
```

https://github.com/DataDog/datadog-agent/blob/50a34e2c7356b8db7616d77ba24b5a8f5bf066d7/comp/updater/telemetry/telemetryimpl/telemetry.go#L41
https://github.com/DataDog/datadog-agent/blob/50a34e2c7356b8db7616d77ba24b5a8f5bf066d7/pkg/config/setup/config.go#L1230

This PR should fix it.


Co-authored-by: arthur.bellal <arthur.bellal@datadoghq.com>

Author: arbll

pkg/fleet/installer/telemetry/telemetry.go

@@ -42,6 +42,9 @@ func NewTelemetry(client *http.Client, apiKey string, site string, service strin
 }
 
 func newTelemetry(client *http.Client, apiKey string, site string, service string) *Telemetry {
+	if site == "" {
+		site = "datadoghq.com"
+	}
 	e := &endpoint{
 		Host:   fmt.Sprintf("https://%s.%s", telemetrySubdomain, strings.TrimSpace(site)),
 		APIKey: apiKey,