Return null on empty json string

PHP7 uses the JSOND extension for parsing JSON which now throws an error
on an empty string. The JSON response is checked whether it is empty
before decoding it and returns null for backwards compatibility.

Author: thomaslorentsen

lib/OpenCloud/Common/Http/Message/Formatter.php

@@ -28,6 +28,9 @@ public static function decode(Response $response)
     {
         if (strpos($response->getHeader(Header::CONTENT_TYPE), Mime::JSON) !== false) {
             $string = (string) $response->getBody();
+            if ('' === $string) {
+                return null;
+            }
             $response = json_decode($string);
             self::checkJsonError($string);