fix: prevent type error on choices array with optional keys

Tests/Translation/Extractor/File/Fixture/MyFormType.php

@@ -67,4 +67,6 @@ public function buildForm(FormBuilderInterface $builder, array $options)
             'empty_value' => ['year' => 'form.dueDate.empty.year', 'month' => 'form.dueDate.empty.month', 'day' => 'form.dueDate.empty.day'],
         ]);
     }
+
+    const CHOICES = ['choices' => [null]];
 }

Tests/Translation/Extractor/File/FormExtractorTest.php

@@ -148,6 +148,10 @@ public function testExtract()
         $message->addSource($fileSourceFactory->create($fixtureSplInfo, 67));
         $expected->add($message);
 
+        $message = new Message(0);
+        $message->addSource($fileSourceFactory->create($fixtureSplInfo, -1));
+        $expected->add($message);
+
         $this->assertEquals($expected, $this->extract('MyFormType.php'));
     }
 

Translation/Extractor/File/FormExtractor.php

@@ -241,10 +241,10 @@ protected function parseChoiceNode(Node $item, Node $node, $domain)
             return true;
         }
 
-        foreach ($item->value->items as $subItem) {
+        foreach ($item->value->items as $index => $subItem) {
             $newItem = clone $subItem;
             $newItem->key = $subItem->value;
-            $newItem->value = $subItem->key;
+            $newItem->value = $subItem->key ?? new Node\Scalar\Int_($index);
             $subItem = $newItem;
             $this->parseItem($subItem, $domain);
         }