R1:c0bc45340584

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…