Add comment clarifying handling of null response object

src/Request/Request.php

@@ -244,6 +244,7 @@ private function createRetryDecider(): callable
                 }
             }
 
+            // It's been observed in the wild that sometimes $response is null.  Get a different error message in that case.
             if (is_object($response) && is_object($response->getBody()) && $response->getBody()->getContents() !== '') {
                 $error = $response->getBody()->getContents();
             } elseif (null !== $exception && '' != $exception->getMessage()) {