arcanist error: Passing null to parameter #2 ($subject) of type string is deprecated

Command 'echo '{"constraints": {"slugs":["#nss-reviewers"]}}' | arcanist call-conduit --conduit-uri=https://phabricator.services.mozilla.com/

 project.search --' returned non-zero exit status 255.

In #400 there was error that prompted us to add --trace to arcanist to diagnose it. Now we have the trace:

CompletedProcess(args='echo \'{"constraints": {"slugs":["#nss-reviewers"]}}\' | arcanist call-conduit --conduit-uri=https://phabricator.services.mozilla.com/ project.search --', returncode=255, stdout=b'', stderr=b'[2025-11-19 00:16:41] EXCEPTION: (RuntimeException) preg_match(): Passing null to parameter #2 ($subject) of type string is deprecated at [<arcanist>/src/error/PhutilErrorHandler.php:261]
arcanist()
  #0 PhutilErrorHandler::handleError(integer, string, string, integer)
  #1 preg_match(string, NULL, NULL) called at [<arcanist>/src/future/http/status/HTTPFutureHTTPResponseStatus.php:24]
  #2 HTTPFutureHTTPResponseStatus::__construct(integer, string, array, NULL) called at [<arcanist>/src/future/http/BaseHTTPFuture.php:384]
  #3 BaseHTTPFuture::parseRawHTTPResponse(string) called at [<arcanist>/src/future/http/HTTPSFuture.php:545]
  #4 Future::updateFuture() called at [<arcanist>/src/future/FutureProxy.php:35]
  #5 FutureProxy::isReady() called at [<arcanist>/src/future/Future.php:63]
  #6 Future::updateFuture() called at [<arcanist>/src/future/FutureIterator.php:224]
  #7 FutureIterator::next() called at [<arcanist>/src/future/FutureIterator.php:190]
  #8 FutureIterator::rewind()
  #9 iterator_to_array(FutureIterator) called at [<arcanist>/src/future/FutureIterator.php:84]
  #10 FutureIterator::resolveAll() called at [<arcanist>/src/future/Future.php:47]
  #11 Future::resolve() called at [<arcanist>/src/workflow/ArcanistCallConduitWorkflow.php:60]
  #12 ArcanistCallConduitWorkflow::runWorkflow(PhutilArgumentParser) called at [<arcanist>/src/workflow/ArcanistWorkflow.php:227]
  #13 ArcanistWorkflow::executeWorkflow(PhutilArgumentParser) called at [<arcanist>/src/toolset/ArcanistPhutilWorkflow.php:21]
  #14 ArcanistPhutilWorkflow::execute(PhutilArgumentParser) called at [<arcanist>/src/parser/argument/PhutilArgumentParser.php:492]
  #15 PhutilArgumentParser::parseWorkflowsFull(array) called at [<arcanist>/src/runtime/ArcanistRuntime.php:171]
  #16 ArcanistRuntime::executeCore(array) ca..

Start with arcanist/src/future/http/status/HTTPFutureHTTPResponseStatus.php and the call from arcanist/src/future/http/BaseHTTPFuture.php:384. Reproduce the shown arcanist call with --trace and follow how the response reaches preg_match with a null subject. Done means this deprecation no longer aborts the command; no test file is named in the issue.