fix: answer null instead of 500 when the export marker is missing

tests/Controller/Api/LastUpdateControllerTest.php

@@ -0,0 +1,50 @@
+<?php
+
+declare(strict_types=1);
+
+namespace App\Tests\Controller\Api;
+
+use App\Controller\Api\LastUpdateController;
+use League\Flysystem\FilesystemOperator;
+use PHPUnit\Framework\TestCase;
+use Symfony\Component\DependencyInjection\Container;
+
+class LastUpdateControllerTest extends TestCase
+{
+    public function testReturnsTheMarkerTimestamp(): void
+    {
+        self::assertSame('2026-09-22 11:10:15', $this->timestampFor(
+            "22.09.2026 11:10:15\r\nLetzte \xDCbertragung\r\nAnzahl \xDCbertragene Dateien: 177\r\nVollexport\r\n"
+        ));
+    }
+
+    public function testAMissingMarkerIsNullRatherThanAServerError(): void
+    {
+        self::assertNull($this->timestampFor(null));
+    }
+
+    public function testAnEmptyMarkerIsNull(): void
+    {
+        self::assertNull($this->timestampFor(''));
+    }
+
+    public function testAnUnparseableMarkerIsNull(): void
+    {
+        self::assertNull($this->timestampFor("gestern\r\nLetzte \xDCbertragung\r\nAnzahl \xDCbertragene Dateien: 177\r\n"));
+    }
+
+    private function timestampFor(?string $marker): ?string
+    {
+        $storage = $this->createStub(FilesystemOperator::class);
+        $storage->method('fileExists')->willReturn(null !== $marker);
+        $storage->method('read')->willReturn((string) $marker);
+
+        $controller = new LastUpdateController($storage);
+        $controller->setContainer(new Container());
+
+        $body = json_decode((string) $controller->index()->getContent(), true);
+        self::assertArrayHasKey('timestamp', $body);
+
+        return $body['timestamp'];
+    }
+}