Fix PHP 8.1 "preg_match(null)" exception exporting task list to CSV

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