Issue #3042694 by mikelutz, jibran: [Symfony 4] JSON:API...

Issue #3042694 by mikelutz, jibran: [Symfony 4] JSON:API ResourceResponseSubscriber can pass NULL to Symfony\Component\HttpFoundation\Response::setCharset()

(cherry picked from commit 0df3be0e)

core/modules/jsonapi/src/EventSubscriber/ResourceResponseSubscriber.php

@@ -173,7 +173,9 @@ protected static function flattenResponse(ResourceResponse $response, Request $r
     $final_response->setContent($response->getContent());
     $final_response->setStatusCode($response->getStatusCode());
     $final_response->setProtocolVersion($response->getProtocolVersion());
-    $final_response->setCharset($response->getCharset());
+    if ($charset = $response->getCharset()) {
+      $final_response->setCharset($charset);
+    }
     $final_response->headers = clone $response->headers;
     if ($final_response instanceof CacheableResponseInterface) {
       $final_response->addCacheableDependency($response->getCacheableMetadata());