fix: default participant's nationality to German to avoid missing data

tests/Service/ParticipantPrepopulationServiceTest.php

@@ -107,10 +107,10 @@ class ParticipantPrepopulationServiceTest extends TestCase
         $this->assertSame('Smith', $result->lastName);
         $this->assertSame('[email protected]', $result->email);
 
-        // Optional fields should be null
+        // Optional fields should be null (except nationality which defaults to 'D')
         $this->assertNull($result->title);
         $this->assertNull($result->gender);
-        $this->assertNull($result->nationality);
+        $this->assertSame('D', $result->nationality);
         $this->assertNull($result->mobile);
         $this->assertNull($result->height);
         $this->assertNull($result->weight);