[Mime] Fix serializing uninitialized RawMessage::$message to null

Author: nicolas-grekas

Normalizer/MimeMessageNormalizer.php

@@ -17,6 +17,7 @@
 use Symfony\Component\Mime\Header\UnstructuredHeader;
 use Symfony\Component\Mime\Message;
 use Symfony\Component\Mime\Part\AbstractPart;
+use Symfony\Component\Mime\RawMessage;
 use Symfony\Component\Serializer\SerializerAwareInterface;
 use Symfony\Component\Serializer\SerializerInterface;
 
@@ -63,15 +64,18 @@ public function normalize($object, ?string $format = null, array $context = [])
             return $ret;
         }
 
+        $ret = $this->normalizer->normalize($object, $format, $context);
+
         if ($object instanceof AbstractPart) {
-            $ret = $this->normalizer->normalize($object, $format, $context);
             $ret['class'] = \get_class($object);
             unset($ret['seekable'], $ret['cid'], $ret['handle']);
+        }
 
-            return $ret;
+        if ($object instanceof RawMessage && \array_key_exists('message', $ret) && null === $ret['message']) {
+            unset($ret['message']);
         }
 
-        return $this->normalizer->normalize($object, $format, $context);
+        return $ret;
     }
 
     /**