revert: revert condition change

Reverted the condition to what it was:
(!$body->isSeekable() || $body->getSize())

Why?, I need to understand better what is the usage of this condition before changing. It was breaking some integ tests.

src/Api/Parser/AbstractRestParser.php

@@ -57,7 +57,7 @@ public function __invoke(
 
         $body = $response->getBody();
         if (!$payload
-            && (!$body->isSeekable() || !$body->getSize())
+            && (!$body->isSeekable() || $body->getSize())
             && count($output->getMembers()) > 0
         ) {
             // if no payload was found, then parse the contents of the body

src/Api/Parser/QueryParser.php

@@ -41,7 +41,7 @@ public function __invoke(
     ) {
         $output = $this->api->getOperation($command->getName())->getOutput();
         $body = $response->getBody();
-        $xml = (!$body->isSeekable() || !$body->getSize())
+        $xml = (!$body->isSeekable() || $body->getSize())
             ? $this->parseXml($body, $response)
             : null;