fix the bug of 204 code with content json back

src/Exception/HttpException.php

@@ -57,6 +57,9 @@ class HttpException extends RuntimeException
 
     public function getBody()
     {
+        if($this->getCode()==204){
+            return null;
+        }
         return json_encode([
             'jsonapi' => ['version' => '1.0'],
             'error' => $this->getCode(),