Avoids a warning about the undefined variable in the case file_get_contents fails

Author: Chikashi-Kato

lib/recurly/http_adapter.php

@@ -52,6 +52,6 @@ public function execute($method, $url, $body, $headers): array
                 }
             }
         }
-        return [$result, $http_response_header];
+        return [$result, $http_response_header ?? null];
     }
 }