Error on SLIM 4

When performing any valid request error ```{"errors":[{"message":"GraphQL Request must include at least one of those two parameters: \"query\" or \"queryId\"","extensions":{"category":"request"}}]}``` is returned.
I've narrowed this to $request->getParsedBody() returning empty array instead of null in slim 4 framework when parsed body is empty. This will fix this issue.

Author: marhub

src/Http/WebonyxGraphqlMiddleware.php

@@ -76,7 +76,7 @@ public function process(ServerRequestInterface $request, RequestHandlerInterface
         }
 
         // Let's json deserialize if this is not already done.
-        if ($request->getParsedBody() === null) {
+        if (empty($request->getParsedBody())) {
             $content = $request->getBody()->getContents();
             $data = json_decode($content, true);