Dashboard Panels: Fix PHP 8.1 "preg_match(null)" exception querying panels

Summary:
Passing null to `preg_match()` is deprecated since PHP 8.1.

```
ERROR 8192: preg_match(): Passing null to parameter #2 ($subject) of type string is deprecated at [/var/www/html/phorge/phorge/src/applications/dashboard/typeahead/PhabricatorDashboardPanelDatasource.php:31]
```

Closes T16421

Test Plan:
* PHP 8.1+
* Create a Dashboard at http://phorge.localhost/dashboard/
* Click "Add Existing Panel"
* In the "Add Panel" overlay click the magnifier button of the "Panel" input field

Reviewers: O1 Blessed Committers, mainframe98

Reviewed By: O1 Blessed Committers, mainframe98

Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T16421

Differential Revision: https://we.phorge.it/D26632

src/applications/dashboard/typeahead/PhabricatorDashboardPanelDatasource.php

···
     $query = new PhabricatorDashboardPanelQuery();
 
     $raw_query = $this->getRawQuery();
-    if (preg_match('/^[wW]\d+\z/', $raw_query)) {
+    if ($raw_query && preg_match('/^[wW]\d+\z/', $raw_query)) {
       $id = trim($raw_query, 'wW');
       $id = (int)$id;
       $query->withIDs(array($id));