R1:79464882da14

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…