Fix error in discovery (librenms#18820)

Device can be null

Author: murrant

app/Jobs/DiscoverDevice.php

@@ -164,6 +164,6 @@ private function handleOsChange(OS $os): OS
 
     public function __destruct()
     {
-        $this->device->save(); // make sure device changes are saved
+        $this->device?->save(); // make sure device changes are saved
     }
 }