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