Fix PHP 8.1 "addcslashes(null)" exception exporting task list to tab-separated text

Summary:
When a column value to export to Tab-Separated Text is empty, null is passed to addcslashes() which is deprecated behavior since PHP 8.1.
Thus only call addclashes() when the value is set.

ERROR 8192: addcslashes(): Passing null to parameter #1 ($string) of type string is deprecated at [/var/www/html/phorge/phorge/src/infrastructure/export/format/PhabricatorTextExportFormat.php:45]

Closes T15771

Test Plan: Export a Maniphest task list of query results to Tab-Separated Text.

Reviewers: O1 Blessed Committers, 20after4, speck

Reviewed By: O1 Blessed Committers, 20after4, speck

Subscribers: avivey, speck, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15771

Differential Revision: https://we.phorge.it/D25568