preg_match() does not accept passing null as the $subject string parameter in PHP 8.1.
Thus add a phutil_nonempty_string() check if the $subject parameter is a non-empty string.
Note: this may highlight other absurd input values that might be worth correcting
instead of just ignoring. If phutil_nonempty_string() throws an exception in your
instance, report it to Phorge to evaluate and fix that specific corner case.
EXCEPTION: (RuntimeException) preg_match(): Passing null to parameter #2 ($subject) of type string is deprecated at [<arcanist>/src/error/PhutilErrorHandler.php:261] arcanist(head=arcpatch-D25250, ref.master=18554ea76ceb, ref.arcpatch-D25250=fade4603a799), phorge(head=diffusionEmptyBranchHistory, ref.master=e11c5486c92b, ref.diffusionEmptyBranchHistory=76f042e4b969) #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/diffusion/request/DiffusionGitRequest.php:6]
Closes T15435