R1:1dd9609beddb

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.