use the empty string instead of null as an array offset

Author: xabbuh

Tests/Fixtures/FixedDataTransformer.php

@@ -25,6 +25,7 @@ public function __construct(array $mapping)
 
     public function transform($value): mixed
     {
+        $value ??= '';
         if (!\array_key_exists($value, $this->mapping)) {
             throw new TransformationFailedException(sprintf('No mapping for value "%s"', $value));
         }

Tests/Fixtures/FixedFilterListener.php

@@ -50,8 +50,8 @@ public function preSetData(FormEvent $event)
     {
         $data = $event->getData();
 
-        if (isset($this->mapping['preSetData'][$data])) {
-            $event->setData($this->mapping['preSetData'][$data]);
+        if (isset($this->mapping['preSetData'][$data ?? ''])) {
+            $event->setData($this->mapping['preSetData'][$data ?? '']);
         }
     }