fix: guard against null return from DataType::export() (#68)

The DataType contract declares export() as ?array, but the Exporter
did not handle a null return — causing a PHP warning when iterating
over it. Skip null returns instead of attempting to foreach them.

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: imorland

src/Exporter.php

@@ -57,6 +57,10 @@ public function export(User $user, User $actor): Export
 
             $data = $segment->export();
 
+            if ($data === null) {
+                continue;
+            }
+
             // Check if the array is an indexed array of associative arrays
             if (is_array($data) && array_values($data) === $data) {
                 // Handling list of associative arrays