fix(stream): ensure .Close() doesn't panic (#201)

Author: stainless-app[bot]

packages/ssestream/ssestream.go

@@ -172,6 +172,7 @@ func (s *Stream[T]) Err() error {
 
 func (s *Stream[T]) Close() error {
 	if s.decoder == nil {
+		// already closed
 		return nil
 	}
 	return s.decoder.Close()