When the Content-Type HTTP header is empty or missing, null is passed to preg_match() which is deprecated behavior since PHP 8.1. Thus only call preg_match() when the value is set.
ERROR 8192: preg_match(): Passing null to parameter #2 ($subject) of type string is deprecated at [$HOME/arcanist/src/future/http/status/HTTPFutureHTTPResponseStatus.php:24]
Closes T15821