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

src/infrastructure/export/format/PhabricatorTextExportFormat.php

@@ -42,8 +42,10 @@ final class PhabricatorTextExportFormat
   private function addRow(array $values) {
     $row = array();
     foreach ($values as $value) {
+      if (phutil_nonempty_string($value)) {
         $row[] = addcslashes($value, "\0..\37\\\177..\377");
       }
+    }
 
     $this->rows[] = implode("\t", $row);
   }