Fix PHP 8.1 "preg_match(null)" exception which blocks rendering the "Browse Herald Rules" dialog

Summary:
`preg_match()` does not accept passing null as the `$subject` string parameter in PHP 8.1.

Thus first check that `$subject !== null`.

```
EXCEPTION: (RuntimeException) preg_match(): Passing null to parameter #2 ($subject) of type string is deprecated at [<arcanist>/src/error/PhutilErrorHandler.php:261]
arcanist(head=master, ref.master=0e32dbc1ac8f), phorge(head=diffusionRepoPage, ref.master=5405134fa5db, ref.diffusionRepoPage=dbe5e3a68c41)
  #0 <#2> PhutilErrorHandler::handleError(integer, string, string, integer) called at [<arcanist>/src/error/PhutilErrorHandler.php:261]
  #1 <#2> preg_match(string, NULL) called at [<phorge>/src/applications/herald/typeahead/HeraldRuleDatasource.php:25]
```
Closes T15422

Test Plan: Applied this change; afterwards on the "Diffusion 🡒 Push Logs 🡒 Advanced Search" page at `/diffusion/pushlog/?repositories=PHID-REPO-someRepositoryString`, clicking the search icon for the "Blocked By" field correctly renders.the "Browse Herald Rules" overlay dialog, listing available Herald rules.

Reviewers: O1 Blessed Committers, valerio.bozzolan

Reviewed By: O1 Blessed Committers, valerio.bozzolan

Subscribers: speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15422

Differential Revision: https://we.phorge.it/D25248

src/applications/herald/typeahead/HeraldRuleDatasource.php

@@ -22,7 +22,7 @@ final class HeraldRuleDatasource
     $query = id(new HeraldRuleQuery())
       ->setViewer($viewer);
 
-    if (preg_match('/^[hH]\d+\z/', $raw_query)) {
+    if (($raw_query !== null) && preg_match('/^[hH]\d+\z/', $raw_query)) {
       $id = trim($raw_query, 'hH');
       $id = (int)$id;
       $query->withIDs(array($id));