Summary: 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 Test Plan: Export a Maniphest task list of query results to CSV. Reviewers: O1 Blessed Committers, 20after4 Reviewed By: O1 Blessed Committers, 20after4 Subscribers: tobiaswiese, valerio.bozzolan, Matthew, Cigaryno Maniphest Tasks: T15770 Differential Revision: https://we.phorge.it/D25567