chore(search): ES-5745 Prevent null message passed to ServerError

Author: jfugalde

src/Bigcommerce/Api/Connection.php

@@ -330,7 +330,7 @@ private function handleResponse()
             }
         } elseif ($status >= 500 && $status <= 599) {
             if ($this->failOnError) {
-                throw new ServerError($body, $status, $this->responseHeaders);
+                throw new ServerError($body ?? '', $status, $this->responseHeaders);
             } else {
                 $this->lastError = $body;
                 return false;