When a column value to export to CSV is empty, 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 ($subject) of type string is deprecated at [/var/www/html/phorge/phorge/src/infrastructure/export/format/PhabricatorCSVExportFormat.php:51]
ERROR 8192: preg_match(): Passing null to parameter #2 ($subject) of type string is deprecated at [/var/www/html/phorge/phorge/src/infrastructure/export/format/PhabricatorCSVExportFormat.php:55]
Closes T15770