fix(symfony): handle empty content-type as set by Symfony (#6078)

* fix: requests with an empty content type
In PHP, requests without content-type will result in a content-type
header containing an empty string, not null

* test: add tests for content type containing an empty string
* test: added a test for DeserializeListener

Author: priyadi

EventListener/DeserializeListener.php

@@ -132,7 +132,7 @@ private function getFormat(Request $request, array $formats): string
     {
         /** @var ?string $contentType */
         $contentType = $request->headers->get('CONTENT_TYPE');
-        if (null === $contentType) {
+        if (null === $contentType || '' === $contentType) {
             throw new UnsupportedMediaTypeHttpException('The "Content-Type" header must exist.');
         }