Fix downtime in corner cases (librenms#16040)

If somehow device outage wasn't recorded, fall back to last_polled (or now)
This should not happen in normal operation

fixes librenms#15634

app/Models/Device.php

@@ -279,7 +279,13 @@ public function getCurrentOutage(): ?DeviceOutage
      */
     public function downSince(): Carbon
     {
-        return Carbon::createFromTimestamp((int) $this->getCurrentOutage()?->going_down);
+        $deviceOutage = $this->getCurrentOutage();
+
+        if ($deviceOutage) {
+            return Carbon::createFromTimestamp((int) $deviceOutage->going_down);
+        }
+
+        return $this->last_polled ?? Carbon::now();
     }
 
     /**