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
c0bc453405