Fix for issue in logs

This fix address the issue described here: https://github.com/louislam/uptime-kuma/issues/1024

server/prometheus.js

@@ -60,7 +60,9 @@ class Prometheus {
             }
 
             try {
-                monitor_cert_days_remaining.set(this.monitorLabelValues, tlsInfo.certInfo.daysRemaining);
+                if ( tlsInfo.certInfo != null ){
+                    monitor_cert_days_remaining.set(this.monitorLabelValues, tlsInfo.certInfo.daysRemaining);
+                }
             } catch (e) {
                 console.error(e);
             }