Don't attempt to show prometheus metrics if URL is empty (#3152)

Signed-off-by: Daniel Valdivia <18384552+dvaldivia@users.noreply.github.com>

restapi/admin_info.go

@@ -881,7 +881,10 @@ func getAdminInfoResponse(session *models.Principal, params systemApi.AdminInfoP
 	prometheusURL := ""
 
 	if !*params.DefaultOnly {
-		prometheusURL = getPrometheusURL()
+		promURL := getPrometheusURL()
+		if promURL != "" {
+			prometheusURL = promURL
+		}
 	}
 
 	mAdmin, err := NewMinioAdminClient(params.HTTPRequest.Context(), session)