Return early on condition if $key is null

This change fixes the situation whenever a call to `array_key_exists` with `$key` set to `null` would trigger a deprecation notice.

Author: agustingomes

src/Formatter/ArrayBasedDeformatter.php

@@ -236,7 +236,7 @@ public function deserializeObject(mixed $decoded, Field $field, Deserializer $de
             first(static fn (string $name): bool => isset($decoded[$name]))
         );
 
-        if (!array_key_exists($key, $decoded)) {
+        if ($key === null || !array_key_exists($key, $decoded)) {
             return DeformatterResult::Missing;
         }