Fix links to non-existent devices (#12680)

Was throwing an exception before instead of returning an empty string

LibreNMS/Util/Url.php

@@ -47,7 +47,7 @@ class Url
      */
     public static function deviceLink($device, $text = null, $vars = [], $start = 0, $end = 0, $escape_text = 1, $overlib = 1)
     {
-        if (is_null($device)) {
+        if (! $device instanceof Device || ! $device->hostname) {
             return '';
         }